JP5831772B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material and a lithium secondary battery including the positive electrode active material.

  A lithium secondary battery is used as a power source for mounting on a vehicle or a power source for a personal computer or a portable terminal. In particular, lithium-ion secondary batteries that are lightweight and provide high energy density are becoming increasingly important as high-output power supplies for vehicles (for example, power supplies that drive motors connected to vehicle drive wheels). ing.

  Lithium secondary batteries are provided with a material (active material) capable of reversibly occluding and releasing lithium ions in the positive and negative electrodes, and charging and discharging are performed by the movement of lithium ions between the positive and negative electrodes. Is called. A typical example of an active material (positive electrode active material) used for the positive electrode is a composite oxide (hereinafter also referred to as a lithium transition metal oxide) containing lithium (Li) and at least one transition metal element. For example, Patent Document 1 describes a non-aqueous electrolyte secondary battery using, as a positive electrode active material, a lithium transition metal oxide in which the particle shape of primary particles and the particle size of secondary particles satisfy a predetermined relationship. . This Patent Document 1 also discloses that the half-value width FWHM of the diffraction peak at the (003) plane with respect to the half-width FWHM (104) of the diffraction peak at the (104) plane in the powder X-ray diffraction pattern of the above oxide ( 003) (FWHM (003) / FWHM (104)) is preferably 0.7 or more and 0.9 or less.

JP 2000-243394 A

  If a lithium secondary battery is used in a wider SOC (state of charge) width, the amount of energy that can be effectively utilized by taking out the unit volume or unit mass of the battery can be increased. This is particularly significant in, for example, a vehicle-mounted battery (for example, a vehicle driving power supply battery) that requires high output and high energy density. However, in general, when the SOC of the lithium secondary battery is low (for example, about 30%), the output (particularly, the output at a low temperature of 0 ° C. or less (for example, about −30 ° C.); hereinafter, also referred to as “low temperature and low SOC output”). Tend to be smaller. The non-aqueous electrolyte secondary battery described in Patent Document 1 also has room for improvement with respect to an improvement in output in a low SOC region.

  If a lithium secondary battery capable of exhibiting a required output even in a low SOC region is provided, the running performance of a vehicle such as a hybrid vehicle or an electric vehicle can be improved. Moreover, if a required output can be exhibited even in a low SOC range, the number of batteries for securing a necessary amount of energy can be reduced. This is advantageous from the viewpoints of cost reduction, weight reduction, and battery mounting space reduction of a product (for example, a vehicle) including a lithium secondary battery. The present invention has been made in view of such circumstances, and an object thereof is to provide a lithium secondary battery with improved output in a low SOC region. Another related object is to provide a positive electrode active material suitable for the positive electrode material and other uses of the lithium secondary battery, and a method for producing the same.

According to the present invention, a lithium secondary battery including a positive electrode and a negative electrode is provided. The positive electrode includes a positive electrode active material having a hollow structure having a shell portion and a hollow portion formed therein. The positive electrode active material includes a lithium transition metal oxide having a layered crystal structure, and the lithium transition metal oxide includes at least one metal element M of nickel (Ni), cobalt (Co), and manganese (Mn). _{Contains T.} In the powder X-ray diffraction pattern of the positive electrode active material, the positive electrode active material is a ratio of the half width A of the diffraction peak obtained by the (003) plane to the half width B of the diffraction peak obtained by the (104) plane ( A / B) is 0.7 or less (for example, 0.4 or more and 0.7 or less). The positive electrode active material has a content of a compound containing Li and CO ₃ (hereinafter sometimes referred to as “Li-CO ₃ compound”) of 0% by mass to 0.2% by mass. .

The lithium transition metal oxide having a layered structure in which the half width ratio (A / B) is 0.7 or less is higher than the lithium transition metal oxide having a larger half width ratio (A / B). Occlusion and release can be performed efficiently. The reason is considered as follows, for example.
In general, a lithium transition metal oxide having a layered structure (for example, LiNiO ₂ ) has a laminated structure in which a lithium layer, an oxygen layer, a transition metal layer, and an oxygen layer are repeatedly stacked, and the direction in which the layers are stacked (c-axis) Li ions are occluded and released from a direction orthogonal to (). If the thickness of the crystal in the direction perpendicular to the c-axis is too thick, the diffusion distance of Li ions becomes long, so that the ion diffusion into the crystal becomes slow. This is disadvantageous for the output performance of the battery. In particular, in the low SOC region, the Li ion concentration in the active material is high, and ion diffusion into the active material becomes rate-limiting during discharge. Therefore, if the ion diffusion is slow, the output characteristics tend to deteriorate.

The lithium transition metal oxide having a layered structure in which the half width ratio (A / B) is 0.7 or less is higher in crystal c-axis than the lithium transition metal oxide having a larger half width ratio (A / B). The thickness in the direction orthogonal to is thin. For this reason, Li ions can diffuse faster in the crystal. This can effectively contribute to an improvement in output in a low SOC region of a lithium secondary battery including the lithium transition metal oxide as a positive electrode active material. In addition, since the positive electrode active material has a hollow structure, the material exchange (for example, movement of Li ions) with the non-aqueous electrolyte can be performed more efficiently than the positive electrode active material having a solid structure. it can. Furthermore, since the content of the Li—CO ₃ compound is suppressed to 0.2% by mass or less in the positive electrode active material, this Li—CO ₃ compound (typically lithium carbonate (Li ₂ CO ₃ )) (For example, the compound prevents Li ions from entering and exiting the lithium transition metal oxide, or prevents the electrolyte from flowing between the hollow portion and the outside.) Event) can be prevented or mitigated. Combined with these effects, the lithium secondary battery having the positive electrode active material in the positive electrode has improved output characteristics (particularly, output in a low SOC region), and exhibits a required output in a wider SOC range. Can be.

In the present specification, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged / discharged by the movement of charges accompanying the lithium ions between the positive and negative electrodes. A battery generally called a lithium ion secondary battery is a typical example included in the lithium secondary battery in this specification. In this specification, “active material” refers to reversibly occlusion and release (typically insertion and desorption) of a chemical species (mainly lithium ion in a lithium secondary battery) that serves as a charge carrier in a secondary battery. A possible substance. Further, in this specification, “SOC” refers to a state of charge of a battery based on a voltage range in which the battery is normally used unless otherwise specified. For example, the rated capacity (typically of an evaluation test battery to be described later) measured under the condition that the terminal voltage (open circuit voltage (OCV)) is 4.1 V (upper limit voltage) to 3.0 V (lower limit voltage). It shall mean the state of charge based on the rated capacity specified under the same conditions as the rated capacity measurement. Further, in this specification, the “positive electrode active material having a hollow structure” means the ratio of the hollow portion in the apparent cross-sectional area of the active material in the average of the cross sections obtained by cutting the positive electrode active material at random positions ( A positive electrode active material having a particle porosity (described later) of 5% or more is used.
The SOC of the battery after the start of use can be grasped, for example, through the following method (SOC-OCV measurement).
[Step 1: Discharge of remaining capacity] At 25 ° C., CCV discharge is performed at 1 C for 2 hours and cut at 3 V.
[Step 2: CC Charging] For the battery after Step 1, the operation of charging at 0.1 C for 30 minutes and the operation of stopping for 20 minutes are repeated 2-3 times, and the battery is cut at 4.1 V.
[Step 3: CCCV charge] The battery after Step 2 is CCV charged at 0.4 C for 1 hour and rested for 20 minutes.
[Step 4: CC Discharge] For the battery after Step 3, the operation of discharging at 0.1 C for 30 minutes and the operation of stopping for 20 minutes are repeated 2 to 3 times, and the battery is cut at 3.0V.

The technique disclosed herein is a ratio of the number of moles of lithium m _Li contained in the positive electrode active material to the total number of moles m _MT of Ni, Co, and Mn contained in the positive electrode active material (m _Li / m _MT ) Is greater than 1.00 (ie, 1.00 <(m _Li / m _MT )). Thus, according to the composition containing excess Li against _{M T,} as compared with the case _(m Li _/ m _MT) is 1.00 or less, a suitable half-width ratios disclosed herein (A / B) It is easy to obtain a positive electrode active material (for example, a positive electrode active material having a smaller half width ratio (A / B)). Also, excess include Li against M _T, and according to the positive electrode active material containing a small amount of Li-CO ₃ compounds, higher performance (e.g., good output performance) lithium secondary battery can be realized . In a preferred embodiment, (m _Li / m _MT ) is 1.10 or more (that is, 1.10 ≦ (m _Li / m _MT )). According to the positive electrode active material having such a composition and the lithium secondary battery including the active material, the above-described effects can be more effectively exhibited.

The art disclosed herein, the positive electrode active material is tungsten (W), in further comprising aspect at least one metal element M _A is selected from chromium (Cr) and molybdenum (Mo) may be preferably performed. According to such a composition, a positive electrode active material having an appropriate half width ratio (A / B) disclosed herein (for example, a positive electrode having a smaller half width ratio (A / B) as compared with the case where no _MA is contained). Active material) is easily obtained. Content _{m MA} of the _{M A,} the Ni contained in the positive electrode active material, as 100 mol% at a molar percentage of total number of moles _{m MT} of Co and Mn, in the range of 0.05 mol% to 1 mol% It is preferable. If the content of M _A is too small, the effect of the inclusion of M _A is may be smaller. If the content of M _A is too large, it is possible that the internal resistance of the battery tends to increase.

As a preferred example of the positive electrode active material in the art disclosed herein, the lithium transition metal oxide is Ni, the positive electrode active material containing Co and Mn be mentioned as the M _T. The lithium transition metal oxide, the composition comprising an excess of Li relative to _{M T (e.g., 1.10 ≦ (m Li / m} MT) in a composition) preferably have a advantage of easy realized.

The positive electrode active material preferably has a through-hole penetrating the shell. Such a positive electrode active material is preferable because the hollow portion can be easily impregnated with a non-aqueous electrolyte, and the productivity of a battery using the positive electrode active material is easily increased. Moreover, in the positive electrode active material having the structure having the through hole, it is particularly meaningful to suppress the content of the Li—CO ₃ compound to a preferable range disclosed herein. Thereby, for example, an event that the through hole is blocked or narrowed by the Li—CO ₃ compound can be avoided, and the effect of having the through hole can be exhibited better.

According to the present invention, a method for producing a positive electrode active material having a hollow structure having a shell and a hollow formed therein is provided. The positive electrode active material that includes a lithium transition metal oxide having a layered crystal structure, the lithium transition metal oxide contains at least one metallic element M _T of Ni, Co and Mn, the (104) plane The ratio (A / B) of the half-value width A of the diffraction peak obtained by the (003) plane to the half-value width B of the obtained diffraction peak is 0.7 or less, and the content of the Li—CO ₃ compound is 0.2. It can be up to mass%. The positive electrode active material manufacturing method typically includes a precursor hydroxide preparation step of preparing a precursor hydroxide containing (a) the M _T. The production method may include (b) a mixing step in which the precursor hydroxide and a lithium compound (for example, lithium carbonate) are mixed to prepare an unfired mixture. Moreover, the said manufacturing method can include the baking process of baking the said mixture and obtaining the said positive electrode active material. Here, the firing step (c) includes (c1) a first firing step in which the mixture is held at the maximum firing temperature of 850 ° C. to 950 ° C. for 1 hour to 12 hours, and (c2) ) A second firing stage in which the result of the first firing stage is held at the maximum firing temperature of 650 ° C. to 800 ° C. for 1 hour to 12 hours. Such a method can be preferably employed as a method for producing any positive electrode active material disclosed herein (which can be any positive electrode active material included in any lithium secondary battery disclosed herein).

In a preferred embodiment of the method disclosed herein, in the mixing step (b), the ratio of the molar number m _Li of lithium to the total molar number m _MT of Ni, Co and Mn (m _Li / m _MT ) is 1. Prepare the green mixture greater than 00 (ie, a mixture where 1.00 <(m _Li / m _MT )). According to this aspect, it is easy to obtain a positive electrode active material having an appropriate half width ratio (A / B) disclosed herein. For example, a positive electrode active material having a smaller half width ratio (A / B) can be produced as compared with the case of (m _Li / m _MT ) ≦ 1.00. Usually, in the mixing step, it is more preferable to prepare an unfired mixture satisfying 1.10 ≦ (m _Li / m _MT ). According to this aspect, the above-mentioned effect can be exhibited better.

As the precursor hydroxides, W, may be at least preferably used those further comprising one metal element M _A is selected from Cr and Mo. By employing such a precursor hydroxide, it becomes easy to obtain a positive electrode active material having an appropriate half width ratio disclosed herein. For example, in comparison with the case of using the precursor hydroxide containing no M _A, smaller positive electrode active material of the half-width ratio (A / B) it can be produced. Content _{m MA} of the _{M A,} the Ni contained in the precursor hydroxide, the total mole number _{m MT} of Co and Mn as 100 mol% in mole percentage in the range of 0.05 mol% to 1 mol% It is preferable that it exists in. If the content of M _A is too small, the effect of the inclusion of M _A is may be smaller. If the content of M _A is too large, it is possible that the internal resistance of the battery tends to increase.

In a preferred embodiment of the method disclosed herein, the precursor hydroxide preparation step (a) comprises:
(A1) in alkaline conditions, and the nucleation step of precipitating the hydroxide from an aqueous solution containing the M _T,
(A2) a particle growth stage for growing the precipitated hydroxide while maintaining the aqueous solution at an alkaline pH lower than that of the nucleation stage;
The hydroxide production | generation process containing is provided. According to the precursor hydroxide obtained by the step (a), a positive electrode active material having a hollow structure can be more accurately produced.

  As another aspect of the present invention, a positive electrode active material produced by any of the methods disclosed herein is provided. Moreover, the positive electrode provided with this positive electrode active material and the non-aqueous secondary battery constructed | assembled using this positive electrode are provided. Here, the “non-aqueous secondary battery” refers to a secondary battery including a non-aqueous electrolyte (typically, an electrolyte that exhibits a liquid state at room temperature (for example, 25 ° C.), that is, an electrolytic solution). A typical example of such a non-aqueous secondary battery is a lithium secondary battery (for example, a lithium ion secondary battery).

  The lithium secondary battery disclosed herein can exhibit good output characteristics even in a low SOC range. Therefore, taking advantage of this feature, it can be suitably used as a drive power source for vehicles such as plug-in hybrid vehicles (PHV) and electric vehicles (EV). The matter disclosed by this specification provides a vehicle equipped with any of the lithium secondary batteries disclosed herein (which may be in the form of an assembled battery in which a plurality of batteries are connected). As a suitable example of such a vehicle, a vehicle (for example, PHV, EV, etc. that can be charged by a household power source) includes the lithium secondary battery as a power source.

It is sectional drawing which shows typically the positive electrode active material particle used for one Embodiment. It is a figure which shows an example of the powder X-ray diffraction pattern of the lithium transition metal oxide which has a layered crystal structure. It is a flowchart which shows the outline of the positive electrode active material manufacturing method which concerns on one Embodiment. It is a perspective view which shows typically one structural example of a lithium secondary battery. It is the VV sectional view taken on the line of FIG. 3 is a temperature profile of a firing pattern I. It is a temperature profile of the baking pattern II. It is a temperature profile of the baking pattern III. It is a graph which shows the relationship between a half value width ratio (A / B) and a low SOC output. It is a side view which shows typically the vehicle carrying a lithium secondary battery.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings. Each drawing is drawn schematically and does not necessarily reflect the real thing. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. Further, in the following drawings, members / parts having the same action are described with the same reference numerals, and overlapping descriptions may be omitted or simplified.

≪Basic composition of positive electrode active material≫
The positive electrode active material in the technology disclosed herein has a hollow structure including a shell portion and a hollow portion formed therein. The positive electrode active material includes a lithium transition metal oxide having a layered crystal structure (typically a layered rock salt structure belonging to a hexagonal system). The lithium transition metal oxide containing a metal element M _T. The _{M T} is at least one of a Ni, Co and Mn. The total content of Ni, Co and Mn in the positive electrode active material (i.e., the content of M _T) is 100 mol% the total amount of all metal elements M _all other than lithium contained in the positive electrode active material in mole percentage Then, for example, it may be 85 mol% or more (preferably 90 mol% or more, typically 95 mol% or more). The positive electrode active material having the composition described above M _T contains at least Ni is preferable. For example, a positive electrode active material containing Ni in an amount of 10 mol% or more (more preferably 20 mol% or more) is preferable, where the total amount of metal elements other than lithium contained in the positive electrode active material is 100 mol%. The positive electrode active material having such a composition is suitable for producing positive electrode active material particles having a hollow structure by applying a production method described later, and therefore, positive electrode active material particles having a desired structure can be easily obtained.

As a preferred example of the lithium transition metal oxide, the M _T is Ni, lithium transition metal oxide containing all of Co and Mn (sometimes to hereinafter referred to as "LNCM oxide".) And the like. For example, the atomic basis, Ni, the total amount of Co and Mn (i.e. the amount of _{M T (m MT))} as 1, Ni, 0.7 more than the amount of Co and Mn is more than 0 none (e.g. LNCM oxide that is more than 0.1 and less than or equal to 0.6, typically more than 0.3 and less than or equal to 0.5 can be preferably employed. Lithium transition metal oxides such ternary (LNCM oxide) is of a composition containing excess Li against _{M T} (i.e., LNCM oxide satisfying _{1.00 <(m Li / m MT} )) Is preferable because it is easy to synthesize. This is presumably because the LNCM oxide has a property of easily taking an excessive amount of Li into the transition metal layer in the above-described laminated structure. In one preferred embodiment, the amount of M _T (atomic number basis) as 1, is in an amount that generally comparable amount and Mn of Ni (e.g., the difference between the amount and the amount of Mn and Ni is 0.1 or less) . For example, a composition in which the amounts of Ni, Co, and Mn are approximately the same can be preferably used. An LNCM oxide having such a composition is preferable because it exhibits excellent thermal stability as a positive electrode active material.

In addition to M _T (that is, at least one of Ni, Co, and Mn), the positive electrode active material in the technology disclosed herein includes one or more other elements as additional constituent elements (additive elements) May be contained. Such additional elements include Group 1 (alkali metals such as sodium), Group 2 (alkaline earth metals such as magnesium and calcium), Group 4 (transition metals such as titanium and zirconium), Group 6 ( Any of those belonging to group 8 (transition metals such as chromium and tungsten), group 8 (transition metals such as iron), group 13 (metals such as boron or a metalloid element) and group 17 (halogens such as fluorine) Can be included. Typical examples include W, Cr, Mo, Zr, Mg, Ca, Na, Fe, Zn, Si, Sn, Al, B, and F. The total content of these additional constituent element is suitable to be less than 20 mole% of M _T, preferably in the ratio of 10 mol% or less.

Positive electrode active material according to the preferred embodiment comprises as the additional element, W, at least one metal element M _A is selected from Cr and Mo. Composition containing such a metal element M _A is a positive electrode active material easily prepared in the preferred ranges FWHM ratio described above (A / B) are disclosed herein. For example, M _T, by firing a mixture comprising M _A and Li to prepare a positive electrode active material (to produce a lithium transition metal oxide) if, at the time of the firing, c-axis of the crystal of the lithium-transition metal oxide The growth in the direction perpendicular to the width is appropriately suppressed, and the half width ratio (A / B) can be adjusted to a preferable range. In particular, the positive electrode active material composition including at least W as M _A is preferred. By including W, in addition to the effect of easily controlling the half-width ratio (A / B) of the positive electrode active material within a preferable range, the effect of reducing the reaction resistance of the battery using the positive electrode active material is exhibited. obtain. This can also contribute to improving the output of the battery.

The positive electrode active material containing the above metal element (additional element) M _A is, for example, a precursor hydroxide containing M _T, a compound containing M _A, by firing a lithium compound, a mixture of (dry mixing Method) can be prepared. Alternatively, it may be prepared by firing a precursor hydroxide composition comprising M _T and M _A, a lithium compound, a mixture of. In the art disclosed herein, it can be preferably used a method of calcining a mixture of a precursor hydroxide and lithium compounds including M _T and M _A. It said precursor hydroxide containing M _T and M _A, for example, can be preferably prepared by the method of producing the precursor hydroxide from the reaction solution containing these (wet method (coprecipitation method)). According to this aspect, in the distribution of M _A in positive electrode active material, it is possible to obtain a positive electrode active material localized agglomeration is prevented. For example, in comparison with the positive electrode active material obtained by the above dry mixing method, the surface of the primary particles of the entire positive electrode active material or active material, it is possible to obtain a more uniform cathode active material was present M _A element .

Content _{m A} of _{M A} (e.g. W) in the cathode active material, the total mole number _{m MT} of _{M T} contained in the positive electrode active material as 100 mol% in mole percentage, for example, a 0.001 mole% In general, it is appropriate to adjust to 0.01 to 3 mol%, and 0.05 to 1 mol% (more preferably 0.1 to 1 mol%, for example 0.2 to 1 mol%). It is preferable that Further, the content of _{m A} of _{M A} (e.g. W) in the positive electrode active material, the total number of moles _{m Mall} of all metal elements _{M all} other than Li contained in the positive electrode active material as 100 mol% in mole percentage, 0 0.001 to 5 mol%, usually 0.01 to 3 mol% is appropriate, 0.05 to 1 mol% (more preferably 0.1 to 1 mol%, for example 0 .2 to 1 mol%). If m _A or m _Mall is too small, the effect of suppressing crystal growth in the c-axis direction tends to be small. If m _A or m _Mall is too large, compared to the positive active material containing no M _A, the reaction resistance of the battery using the positive electrode active material may increase rather.

In a preferred embodiment, the positive electrode active material may have a composition (average composition) represented by the following general formula (I).
Li _{1 + x} Ni _y Co _z Mn _(1-yz) M _Aα M _Bβ O ₂ (I)
In the above formula (I), x may be a real number that satisfies 0 ≦ x ≦ 0.2. y may be a real number that satisfies 0.1 <y <0.6. z may be a real number that satisfies 0.1 <z <0.4. M _A is at least one metal element selected from W, Cr and Mo, and α is 0 <α ≦ 0.01 (typically 0.0005 ≦ α ≦ 0.01, for example 0.001 ≦ It is a real number satisfying α ≦ 0.01). M _B is, Zr, Mg, Ca, Na , Fe, Zn, Si, Sn, Al, and one or more elements are selected from the group consisting of B and F, beta is 0 ≦ β ≦ 0. It can be a real number satisfying 01. β is substantially zero (i.e., substantially free oxide M _B) may be.
Note that in this specification, the chemical formula showing the lithium transition metal oxide having a layered structure shows the composition ratio of O (oxygen) as 2 for convenience, but this numerical value should not be interpreted strictly, Variation in the composition (typically included in the range of 1.95 to 2.05).

≪Half width ratio (A / B) ≫
The positive electrode active material used for the lithium secondary battery according to the present embodiment has a Miller index (with respect to the half width B of the peak obtained by the diffraction surface of the Miller index (104) in a powder X-ray diffraction pattern using CuKα rays. The ratio (A / B) of the half width A of the peak obtained by the diffractive surface of (003) is 0.7 or less (typically less than 0.7). The lithium transition metal oxide exhibiting such a half width ratio (A / B) has a thicker c-axis thickness than the lithium transition metal oxide exhibiting a larger half width ratio (A / B). And the thickness in the direction perpendicular to the c-axis may be thinner. As the thickness in the c-axis direction increases, the number of surfaces into which Li ions can be inserted increases. Further, when the thickness in the direction orthogonal to the c-axis (for example, the a-axis direction) is reduced, the ion diffusion distance in the crystal is shortened. Therefore, according to the positive electrode active material having such a configuration, it is possible to efficiently store and release Li ions. Specifically, since the diffusion of Li ions into the crystal is fast, Li ions are likely to be released from the inside of the crystal during charging, and Li ions are easily absorbed up to the inside of the crystal during discharging. Therefore, by adopting the positive electrode active material exhibiting the half width ratio (A / B), it is possible to effectively improve the output characteristics (especially the output characteristics in the low SOC region) of the lithium secondary battery.

  Here, X-ray diffraction (XRD) measurement using CuKα rays may be performed by making X-rays (CuKα rays) emitted from an X-ray generation source incident on the sample surface of the sample. . The sample surface may be a surface made of a positive electrode active material (typically powdered). For example, a surface (positive electrode active) in which a powdered positive electrode active material is actually bound with a binder to form a positive electrode. The surface of the material layer). At this time, it is preferable to irradiate X-rays while changing the incident angle with respect to the sample stepwise or continuously while rotating and scanning the sample with a predetermined scanning axis, and to capture the X-rays diffracted by the sample with an inspection device. Then, the angle difference (diffraction angle 2θ) between the X-ray diffraction direction and the incident direction and the diffraction X-ray intensity are measured. Such X-ray diffraction measurement can be performed using X-ray diffraction measurement devices commercially available from various measurement device manufacturers. For example, an X-ray diffraction measuring device manufactured by Rigaku Corporation, a product name “MultiFlex” can be used.

As an example, FIG. 2 shows an X-ray diffraction pattern of a lithium transition metal oxide having a layered structure having a composition represented by LiNi _1/3 Co _1/3 Mn _1/3 O ₂ . In the diffraction pattern of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , a diffraction peak derived from the (003) plane occurs near a diffraction angle of 18 ° C. In addition, a diffraction peak derived from the (104) plane occurs near a diffraction angle of 45 ° C. From the diffraction pattern thus obtained, the half-value width A of the diffraction peak derived from the (003) plane and the half-value width B of the diffraction peak derived from the (104) plane may be calculated. The half-value widths A and B can be calculated using analysis software attached to the X-ray diffraction measurement device commercially available from various measurement device manufacturers. For example, “JADE” which is analysis software attached to an X-ray diffraction measurement device manufactured by Rigaku Corporation can be used.

  In the technique disclosed herein, the half width ratio (A / B) is preferably 0.65 or less, more preferably 0.60 or less (typically less than 0.60, for example, 0.58). The following). According to the positive electrode active material exhibiting such a half width ratio (A / B), a lithium secondary battery exhibiting higher output characteristics can be realized. The lower limit of the half width ratio (A / B) is not particularly limited, but usually a positive electrode active material having a half width ratio (A / B) of 0.35 or more (for example, 0.40 or more) is preferable. The positive electrode active material having an excessively small half width ratio (A / B) may be difficult to generate (synthesize) and crystal growth (particularly, growth in a direction perpendicular to the c-axis) may be insufficient. In a battery including such a positive electrode active material, there is a concern that the metal element in the positive electrode active material is likely to elute into the electrolyte during high temperature storage. Such elution of the metal element can cause the battery capacity to deteriorate. From the viewpoint of capacity retention during high temperature storage, the half-width ratio (A / B) of the positive electrode active material is suitably 0.40 or more (for example, 0.40 <(A / B)), 0.50 The above (for example, 0.50 <(A / B)) is preferable. For example, a positive electrode active material satisfying 0.40 ≦ (A / B) <0.70 can be preferably employed from the viewpoint of achieving a balance between output characteristics and capacity retention. Positive electrode active material satisfying 0.40 <(A / B) ≦ 0.65 (further, 0.40 <(A / B) <0.60, for example, 0.50 ≦ (A / B) <0.60) Better results can be achieved.

≪Hollow structure≫
The positive electrode active material in the technology disclosed herein typically has a hollow particle form having a shell and a hollow (cavity) formed therein. The particle shape of the particulate positive electrode active material (positive electrode active material particles) typically can be approximately spherical, slightly distorted spherical, or the like. In a preferred embodiment, the shell part has a through hole that allows the hollow part to communicate with the outside of the particle (hereinafter, the hollow structure having the through hole in the shell part may be referred to as a “perforated hollow structure”). . In addition, as a thing contrasted with the particle | grains of such a hollow structure (it is the meaning which includes a void | hole hollow structure unless it mentions specially), the particle | grains of a general porous structure are mentioned. Here, the porous structure refers to a structure (sponge-like structure) in which a substantial part and a void part are mixed over the entire particle. As a typical example of the active material particles having a porous structure, active material particles obtained by a so-called spray firing method (sometimes referred to as a spray dry production method) can be given. The hollow structure active material particles disclosed herein are substantially the same as the porous structure active material particles in that the substantial part is biased toward the shell and a space is secured in the hollow part. Are clearly distinguished in structure.

  The positive electrode active material according to a preferred embodiment is in the form of secondary particles in which primary particles of the lithium transition metal oxide are collected. Here, the “primary particle” refers to a particle that is considered to be a unit particle (ultimate particle) based on an apparent geometric form. In the positive electrode active material disclosed herein, the primary particles are typically a collection of crystallites of a lithium transition metal oxide. The shape of the positive electrode active material can be observed, for example, by “Hitachi High-Tech Hitachi Ultra High Resolution Electrolytic Emission Scanning Microscope S5500” manufactured by Hitachi High-Technologies Corporation.

  A typical structure of such positive electrode active material particles is schematically shown in FIG. The positive electrode active material particle 110 is a hollow structure particle having a shell portion 115 and a hollow portion 116. The shell portion 115 has a form in which the primary particles 112 are gathered in a spherical shell shape. In a preferred embodiment, the shell 115 has a form in which the primary particles 112 are arranged in a ring (beaded) in a cross-sectional SEM (Scanning Electron Microscope) image. The primary particles 112 may be in a single (single layer) continuous form over the entire shell 115, or two or more primary particles 112 may be stacked (in multiple layers) to have a continuous part. Good. The number of stacked primary particles 112 in the continuous portion is preferably about 5 or less (for example, 2 to 5), and more preferably about 3 or less (for example, 2 to 3). The positive electrode active material particles 110 according to a preferred embodiment are configured such that the primary particles 112 are substantially continuous in a single layer over the entire shell 115.

  The active material particles (secondary particles) 110 having such a configuration are less aggregated of the primary particles 112 than the active material particles having a dense structure without a cavity inside. For this reason, there are few grain boundaries in the particles (as a result, the diffusion distance of Li ions is shorter), and the diffusion of Li ions into the particles is fast. According to such an active material particle 110 with few grain boundaries, the lithium having the particle 110 is coupled with the rapid diffusion of Li ions into the crystal due to the regulation of the half width ratio (A / B) described above. The output characteristics of the secondary battery can be effectively improved. For example, a lithium secondary battery that exhibits good output even in a low SOC region (for example, when the SOC is 30% or less) where ion diffusion into the active material is rate-limiting can be constructed.

≪Through hole≫
The positive electrode active material particle 110 preferably has a through hole 118 that passes through the shell portion 115 and spatially continues the hollow portion 116 and the outside (outside of the particle 110). By having such a through-hole 118, the electrolytic solution easily goes back and forth between the hollow portion 116 and the outside, and the electrolytic solution in the hollow portion 116 is appropriately replaced. For this reason, it is hard to produce withering of the electrolyte solution in the hollow part 116, and the primary particles 112 facing the hollow part 116 can be more actively utilized for charging and discharging. According to such a configuration, the diffusion of Li ions into the crystal is fast due to the above-described half width ratio (A / B), and the electrolyte solution can be efficiently brought into contact with the primary particles 112. Thus, the output characteristics of the lithium secondary battery (especially the output characteristics in the low SOC region) can be further improved.

  The number of through-holes 118 included in the positive electrode active material particles 110 is preferably about 1 to 10 (for example, 1 to 5) as an average per particle of the active material particles 110. If the average number of through holes is too large, it may be difficult to maintain the hollow shape. According to the active material particles 110 having a preferable average number of through-holes disclosed herein, an effect of improving battery performance (for example, an effect of improving output) by having a perforated hollow structure while ensuring the strength of the active material particles 110. Can be satisfactorily and stably exhibited.

The opening width h of the through hole 118 is preferably about 0.01 μm or more as an average value of the plurality of active material particles. Here, the opening width h of the through hole 118 refers to a passing length in the narrowest part in the path from the outside of the active material particle 110 to the hollow part 116. When the opening width of the through holes 118 is 0.01 μm or more on average, the through holes 118 can function more effectively as a flow path for the electrolytic solution. Thereby, the effect which improves the battery performance of a lithium secondary battery can be exhibited more appropriately.
When one active material particle 110 has a plurality of through holes 118, the opening width of the through hole having the largest opening width among the plurality of through holes 118 is adopted as the opening width of the active material particles 110. Good. The opening width h of the through holes 118 may be an average of 2.0 μm or less, more preferably an average of 1.0 μm or less, and even more preferably an average of 0.5 μm or less.

  In addition, characteristic values, such as the said average number of through-holes and average opening size, can be grasped | ascertained by observing the cross section of active material particle | grains by SEM, for example. For example, a sample obtained by solidifying active material particles or a material containing the active material particles with an appropriate resin (preferably a thermosetting resin) is cut in an appropriate cross section, and SEM observation is performed while cutting the cut surface little by little. Good. Or, since it can be assumed that the orientation (posture) of the active material particles is generally random in the sample, the SEM observation results on a single cross section or a relatively small number of cross sections of about 2 to 10 points are statistically calculated. The characteristic value can also be calculated by processing.

  In a preferred embodiment, as schematically shown in FIG. 1, the shell 115 is dense (typically at least not allowing general electrolyte for a nonaqueous battery to pass through) in a portion other than the through-hole 118. Sintered (densely). According to the active material particle 110 having such a structure, a portion where the electrolyte solution can flow between the outside of the particle 110 and the hollow portion 116 is limited to a portion where the through hole 118 is provided. Thereby, for example, particularly advantageous effects can be exhibited in the active material particles used for the positive electrode of a battery including a wound electrode body. That is, in a battery including a wound electrode body, when charging and discharging of the battery is repeated, the electrolyte solution is squeezed out from the electrode body (particularly, the positive electrode active material layer) due to expansion and contraction of the active material accompanying charging and discharging. Battery performance (for example, output performance) may be reduced due to insufficient electrolyte in a part of the body. According to the active material particle 110 having the above-described configuration, the electrolyte solution in the hollow portion 116 is prevented from flowing out in the portion other than the through-hole 118, so that the lack of electrolyte solution in the positive electrode active material layer can be effectively prevented. Or it can be reduced. In addition, since the active material particles can have high shape maintainability (being difficult to collapse; for example, it can be reflected in that the average hardness is high, the compressive strength is high, etc.), good battery performance is more stable. Can be demonstrated.

≪Li-CO ₃ compound amount≫
In the positive electrode active material disclosed herein, the content of a Li—CO ₃ compound (typically lithium carbonate (Li ₂ CO ₃ )) is 0.2% by mass or less. That is, the positive electrode active material does not contain a Li—CO ₃ compound, or contains a Li—CO ₃ compound in an amount not exceeding 0.2 mass% of the positive electrode active material. This Li—CO ₃ compound can be derived from the lithium compound (Li ₂ CO ₃ , LiOH, etc.) used in the step of preparing the positive electrode active material. For example, when a lithium transition metal oxide is produced by heating (firing) a mixture of a transition metal hydroxide as a positive electrode active material precursor and the above lithium compound, an excess of the lithium compound (M _T amount) or minutes using excess relative to M _all), which may be unreacted, side reaction products, or Li-CO ₃ compounds formed by reaction with CO ₂ and the like thereof in the atmosphere (e.g. in air) .

Such a Li—CO ₃ compound is not included in the composition of the lithium transition metal oxide, and may exist mainly on the surface of the oxide (typically the surface of the primary particle or the grain boundary). Therefore, when the Li—CO ₃ compound content of the positive electrode active material is too large, Li existing on the surface of the lithium transition metal oxide crystal contained in the positive electrode active material (particularly, the surface on which Li ions can enter and exit). The —CO ₃ compound can prevent Li ions from entering and exiting the crystal. By suppressing the Li—CO ₃ compound content of the positive electrode active material to a low level, the effect that the positive electrode active material has the preferred half-width ratio (A / B) disclosed herein can be better exhibited. In the lithium secondary battery disclosed herein, for example, the positive electrode active material has a half width ratio (A / B) of less than 0.70 (typically 0.40 or more and less than 0.70), and Li It can be preferably implemented in an embodiment in which the —CO ₃ compound content is 0.2% by mass or less (typically 0.05% by mass or more and 0.20% by mass or less).

Generally, the amount of lithium compound used in the preparation process of the positive electrode active material (e.g., for M _T amount or M _all amounts included in the precursor transition metal hydroxide, the amount of Li contained in the lithium compound) Gayori When it increases, the Li—CO ₃ compound content of the obtained positive electrode active material tends to increase. Containing Therefore, when preparing the precursor mixture is fired lithium transition metal oxide having a composition containing excess Li as described above with transition metal hydroxide and the lithium compound, Li-CO ₃ compound When the above baking is performed without the intention of reducing the amount, the obtained positive electrode active material tends to contain a large amount of Li-CO ₃ compound. In particular, in the production of the positive electrode active material having the hollow structure, a Li—CO ₃ compound (or a Li—CO ₃ compound reacts with CO ₂ or the like in air in the atmosphere) in the hollow portion of the positive electrode active material. The chemical species to be obtained) tends to accumulate, and therefore the Li—CO ₃ compound content of the obtained positive electrode active material tends to increase. In addition, in a positive electrode active material having a hollow structure (perforated hollow structure) having a through hole in the shell portion, when the Li—CO ₃ compound closes the through hole and narrows the opening width, the electrolyte solution flows through the through hole. (For example, liquid diffusion from the outside of the particle to the hollow portion) can be prevented. This can be a factor of reducing the output of the lithium secondary battery including the positive electrode active material. Therefore, in the positive electrode active material having such a structure, it is particularly meaningful to keep the amount of Li—CO ₃ compound low.

The content of the Li—CO ₃ compound is preferably as small as possible from the viewpoint of preventing a decrease in output due to the compound. In the positive electrode active material according to a preferred embodiment, the Li—CO ₃ compound content is 0.18% by mass or less, for example, 0.15% by mass or less. On the other hand, when an attempt is made to prepare a positive electrode active material with a low Li—CO ₃ compound content, the half width ratio (A / B) of the lithium transition metal oxide contained in the positive electrode active material tends to increase. In particular, when the lithium transition metal oxide has a composition containing excessive Li, if the Li—CO ₃ compound content is extremely reduced, the half width ratio (A / B) may become too large. possible. Therefore, the lower limit of the Li—CO ₃ compound content is suitably 0.01% by mass (preferably 0.05% by mass, for example 0.10% by mass). In the technique disclosed herein, the Li—CO ₃ compound content of the positive electrode active material is 0.01 to 0.18% by mass (more preferably 0.05 to 0.18% by mass, for example, 0.10% by mass or more). (Less than 0.15% by mass).

The following method can be preferably used for quantifying the Li—CO ₃ compound content. That is, the positive electrode active material disclosed herein can include an alkali salt (a lithium compound that exhibits alkalinity when dissolved in water) such as lithium hydroxide (LiOH) or lithium oxide (Li ₂ O). In order to accurately measure the Li—CO ₃ compound content of the positive electrode active material even in the presence of such an alkali salt, water and barium chloride are added to the positive electrode active material, and carbonate ions contained in the positive electrode active material are added. After precipitation as barium carbonate (BaCO ₃ ), the lithium alkali salt may be titrated to the first neutralization point (about pH 9) with hydrochloric acid and then titrated to the second neutralization point (about pH 4) with hydrochloric acid. Barium carbonate, in the titration to the first neutralization point does not react, it produces a BaCl ₂ and neutralization reaction with HCl by titration up to the second neutral point. Therefore, the amount of Li—CO ₃ compound can be calculated from the difference in titer between the second neutralization point and the first neutralization point. More specifically, for example, the Li—CO ₃ compound quantification method described in Examples described later can be preferably employed.

Most of the Li—CO ₃ compound contained in the positive electrode active material disclosed herein (typically 70 to 100% by mass, for example, 85 to 100% by mass, and further 95 to 100% by mass) is Li _2. It can be CO ₃ . Li ₂ CO ₃ may be substantially all of the Li—CO ₃ compound contained in the positive electrode active material (for example, 98 to 100 mass%, further 99.5 to 100 mass%).
The positive electrode active material disclosed herein is also a compound capable of forming a water-soluble alkali salt upon contact with a water-soluble alkali salt or water of lithium (lithium hydroxide, lithium oxide, etc .; hereinafter, “water-soluble lithium alkali salt or Also referred to as a “precursor thereof”). In a preferred embodiment, the positive electrode active material does not substantially contain the water-soluble lithium alkali salt or a precursor thereof (for example, the content is 0 to 0.01% by mass).

The positive electrode active material in the art disclosed herein is to prepare a precursor hydroxide containing at least M _T (e.g., prepared by a wet process to produce the precursor hydroxide from the reaction solution containing M _T), The precursor hydroxide is mixed with an appropriate lithium compound (compound that can be a lithium source; for example, a lithium salt such as Li ₂ CO ₃ , LiOH), and the unfired mixture is fired under appropriate conditions. be able to. This firing step is desirably performed in an oxidizing atmosphere (for example, the atmosphere or an atmosphere richer in oxygen than the atmosphere). In a preferred embodiment, the firing step includes a first firing stage (c1) for firing at a maximum firing temperature T1, and a second firing at a maximum firing temperature T2 (that is, T1> T2) lower than the maximum firing temperature T1. And a firing step (c2). By firing the mixture by such a multi-stage firing schedule, the preferred half-width ratio (A / B) disclosed herein and the Li—CO ₃ compound content were both compatible (typically, half A positive active material having a hollow structure in which the valence ratio (A / B) is not more than a predetermined value and the Li—CO ₃ compound content is suppressed to be not more than a predetermined value can be efficiently formed.

  The first firing step (c1) can be grasped mainly as a step of reacting the precursor hydroxide and the lithium compound by heating the unfired mixture. The maximum firing temperature T1 in the first firing stage is a temperature range in which the sintering reaction of the target lithium transition metal oxide can proceed and is a temperature range below the melting point of the lithium transition metal oxide. For example, it can be determined within a range of 800 ° C to 1000 ° C. When the maximum firing temperature T1 exceeds 1000 ° C., the growth (enlargement) of primary particles proceeds remarkably, which may cause a decrease in hollowness (and hence a decrease in battery performance). Usually, it is appropriate that the maximum firing temperature T1 is in the range of 850 ° C. to 950 ° C. (for example, 900 ° C. to 950 ° C.). By setting it as such firing conditions, crystal growth in the direction orthogonal to the c-axis is suppressed during sintering, and the positive electrode active material having the above-described preferred half-width ratio (A / B) can be accurately manufactured. If the maximum firing temperature T1 is too high, crystals may grow too much during sintering, and the full width at half maximum ratio (A / B) may be too larger than the preferred value described above. If the maximum firing temperature T1 is too low, the crystal growth tends to be insufficient, or the time required for the firing process becomes long and the productivity of the positive electrode active material tends to decrease.

  The time t1 held at the maximum firing temperature T1 can be, for example, 0.5 hours to 12 hours, and is preferably 1 hour to 12 hours (for example, 2 hours to 10 hours). When the holding time t1 is too short, crystal growth becomes insufficient, or a large amount of unreacted lithium compound tends to remain. If the holding time t1 is too long, crystals may grow too much during sintering, and the half-width ratio (A / B) may be too larger than the above-described preferred value. In addition, the time required for the firing process becomes long, and the productivity of the positive electrode active material tends to decrease. From the viewpoint of productivity of the positive electrode active material, the holding time t1 is preferably 8 hours or less (more preferably 7 hours or less, for example, 6 hours or less).

The second firing step (c2) mainly reacts with the Li—CO ₃ compound or CO ₂ that may be present in the resultant product while suppressing further crystal growth of the resultant product obtained by the first firing step. It can be understood as a step of reducing the amount of chemical species (typically lithium compounds that are not included in the composition of the lithium transition metal oxide, that is, out-of-composition lithium compounds) that can yield Li-CO ₃ compounds. The decrease in the amount of the out-of-composition lithium compound can be realized by incorporating (absorbing) Li contained in the compound into a lithium transition metal oxide (for example, in a transition metal layer, a lithium layer, or the like). The maximum firing temperature T2 in the second firing stage can be determined, for example, within a range of 600 ° C. to 850 ° C. (where T1> T2), and is usually 650 ° C. to 800 ° C. (eg, 700 ° C. to 800 ° C.). It is appropriate to be within the range. If the maximum firing temperature T2 is too high, the crystal growth proceeds too much before the amount of Li—CO ₃ compound is sufficiently lowered, and the half-width ratio (A / B) becomes too larger than the above-described preferred value. It can happen. If the maximum firing temperature T2 is too low, the time required for sufficiently reducing the amount of the Li—CO ₃ compound becomes long, and the productivity of the positive electrode active material tends to be lowered. In a preferred embodiment, the maximum firing temperature T2 is 700 ° C to 770 ° C (more preferably 700 ° C to 750 ° C, for example, 720 ° C to 740 ° C). According to such maximum firing temperature T2, the amount of Li—CO ₃ compound can be efficiently reduced, and excessive crystal growth (an event in which the full width at half maximum (A / B) becomes too large) can be effectively suppressed.

The time (holding time) t2 for holding at the maximum firing temperature T2 can be, for example, 0.5 hours to 12 hours, and is usually suitably 1 hour to 12 hours (for example, 2 hours to 10 hours). It is. When the holding time t2 is too short, the Li—CO ₃ compound content in the obtained positive electrode active material tends to be larger than the above-described preferable range. If the holding time t2 is too long, the time required for the firing process becomes long, and the productivity of the positive electrode active material tends to decrease. In addition, crystal growth may proceed excessively, and the full width at half maximum ratio (A / B) may be excessively larger than the preferable value described above. From the viewpoint of productivity of the positive electrode active material, the holding time t2 is preferably 8 hours or less (more preferably 6 hours or less, for example, 5 hours or less).

In a preferred embodiment, the firing step can be performed under the condition that the total time of t1 and t2 (t1 + t2) is less than 20 hours (more preferably 15 hours or less, for example, 13 hours or less). According to such an embodiment, a positive electrode active material satisfying the preferred half width ratio (A / B) and Li—CO ₃ compound content disclosed herein can be produced with higher productivity. The lower limit of (t1 + t2) is not particularly limited as long as a positive electrode active material satisfying desired characteristics is obtained, but (t1 + t2) is usually 2 hours or longer (for example, 4 hours or longer).

The result of the first firing stage (the fired product at the end of the first firing stage) preferably satisfies the preferred half width ratio (A / B) (for example, 0.4 to 0.7) disclosed herein. . This is because if the full width at half maximum (A / B) at the end of the first firing stage is too large, it is difficult to significantly reduce it at the second firing stage. Further, the calcined product at the end of the first firing stage is in an amount exceeding 0.2% by mass (preferably more than 0.2% by mass and 1.0% by mass or less, for example, more than 0.2% by mass and less than 0.2% by mass). 5% by mass or less) Li—CO ₃ compound amount is preferably included. For example, when the green mixture is prepared such that (m _Li / m _MT ) or (m _Li / m _Mall ) is larger than 1.00 (for example, 1.10 or more), the first baking stage is completed. In this way, the fired product tends to have a high Li—CO ₃ compound content. According to an embodiment in which the second firing stage is applied to such a fired product to reduce the Li—CO ₃ compound content to 0.2% by mass or less, the significance of employing the production method disclosed herein is particularly significant.

The first firing stage and the second firing stage are continuously performed (for example, after maintaining the unfired mixture at the maximum firing temperature T1 of the first firing stage, the temperature is subsequently raised to the maximum firing temperature T2 of the second firing stage. Or after maintaining at the maximum firing temperature T1 in the first firing stage, and then cooling (for example, cooling to room temperature) and crushing and sieving as necessary. You may use for a 2nd baking step, after dividing. It is appropriate to provide a temperature difference of 50 ° C. or more (typically 100 ° C. or more, for example, 150 ° C. or more) between the maximum firing temperature T1 in the first firing stage and the maximum firing temperature T2 in the second firing stage. It is. According to such an embodiment, the first and second firing stages can more accurately fulfill their roles.
The firing process may include a firing stage other than the first firing stage and the second firing stage as necessary. However, after the completion of the first firing stage, the firing process is performed at a maximum firing temperature T1 or higher. It is preferable not to have a stage that is continuously held for 5 hours or more. This is because, at this stage, crystal growth proceeds and the full width at half maximum (A / B) may become too large.

The method disclosed herein is preferably applied to the production of a positive electrode active material having a composition containing M _A (eg, W) in addition to M _T , and the preferred half width ratio (A / B) and Li—CO ₃ described above A positive electrode active material satisfying the compound content can be provided. In this case, the mixture to be subjected to the burning step as (unfired mixture), a precursor hydroxide composition comprising M _T and M _A, the mixture can be preferably used a lithium compound. Alternatively, a precursor hydroxide containing M _T, a compound containing M _A, may be used a mixture of a lithium compound.

In one preferred embodiment, the mixture ratio of the mole number _{m Li} of Li to moles _{m MT} of _{M T} contained in the mixture _(m Li _{/ m MT)} is prepared to be greater than 1.00 ( That is, a mixture in which 1.00 <(m _Li / m _MT ) is prepared). By firing such a mixture, typically, a positive electrode active material in which (m _Li / m _MT ) is approximately the same value as the above mixture can be formed. Thus, according to the mixture composition containing excess Li against _{M T, (m Li / m} MT) compared with the case of 1.00 or less, a suitable half-width ratios disclosed herein (A / It is easy to obtain a positive electrode active material having B) (for example, a positive electrode active material having a smaller half width ratio (A / B)). This is considered to be because, according to the mixture having such a composition, crystal growth in the direction perpendicular to the c-axis can be suppressed during the firing. Also, excess include Li against M _T, and according to the positive electrode active material is small Li-CO ₃ compound content, higher performance (e.g., good output performance) lithium secondary battery can be realized. In a preferred embodiment, (m _Li / m _MT ) of the mixture or the positive electrode active material is 1.05 or more (that is, 1.05 ≦ (m _Li / m _MT )), more preferably 1.10 or more. It is. According to the positive electrode active material having such a composition and the lithium secondary battery including the active material, the above-described effects can be more effectively exhibited.

The mixture containing Li excessively as described above is preferable from the above viewpoints (in terms of half-width ratio (A / B), battery performance, etc.), while the conventional normal firing mode (typically one stage). ), The Li—CO ₃ compound content tends to be too high. In particular, the positive electrode active material of a hollow structure having a shell portion and a hollow portion, Li-CO ₃ compound to the hollow portion (or becomes Li-CO ₃ compound reacts with CO ₂ and the like in the air in the atmosphere To obtain), the Li-CO ₃ compound content tends to increase. Therefore, in a mode in which a positive electrode active material is obtained by firing a mixture having such a composition, the firing process disclosed herein (the firing process including the first firing stage and the second firing stage as described above) is applied. Is particularly significant. According to such a firing step, even when the mixture containing Li in excess is fired to produce positive electrode active material particles having a hollow structure, the Li—CO ₃ compound content is efficiently reduced, and the preferred half-width A positive electrode active material that simultaneously satisfies the ratio (A / B) and the Li—CO ₃ compound content can be produced.

_The upper limit of the _(m Li _/ m _MT) is not particularly _limited, when the _(m Li _/ m _MT) is too large, a positive electrode active material in the preferred ranges Li-CO ₃ compound content are disclosed herein It tends to be difficult to obtain, or the time required to obtain such a positive electrode active material (for example, the peak retention time in the second firing stage) tends to be long. Therefore, normally, (m _Li / m _MT ) is suitably 1.30 or less (that is, (m _Li / m _MT ) ≦ 1.30), and preferably 1.20 or less.

In another preferred embodiment, the positive electrode active material has a composition containing an excess amount of Li with respect to the total amount m _Mall of all metal elements other than Li contained in the positive electrode active material (that is, 1.00). <(M _Li / m _Mall )). Thus, according to the composition containing Li excessively with respect to M _all , the appropriate half width ratio (A / B) disclosed herein is compared with the case where (m _Li / m _Mall ) is 1.00 or less. It is easy to obtain a positive electrode active material (for example, a positive electrode active material having a smaller half width ratio (A / B)). Also, excess include Li against M _all, and according to the positive electrode active material is small Li-CO ₃ compound content, higher performance (e.g., good output performance) lithium secondary battery can be realized. In a preferred embodiment, (m _Li / m _Mall ) is 1.05 or more, more preferably 1.10 or more (that is, 1.10 ≦ (m _Li / m _Mall )). According to the positive electrode active material having such a composition and the lithium secondary battery including the active material, the above-described effects can be more effectively exhibited.

In the technique disclosed herein, the full width at half maximum ratio (A / B) can be adjusted by various methods. For example, as a method for half-width ratio (A / B) smaller, (1) M _T or more the content of Li to M _all (e.g., in the mixing step described below, the precursor oxide hydroxide (2) lower the maximum firing temperature T1 in the first firing stage; and (3) a time t1 to maintain the maximum firing temperature T1 in the first firing stage. Examples of the method include: making it shorter; using M _A (for example, W) as an additive element; These methods can be employed alone or in appropriate combination.

≪Production of positive electrode active material≫
Hereinafter, a lithium transition metal oxide LNCM oxide, as a main example the preparation of the positive electrode active material composition including W as the metal element M _A, a preferred embodiment of the cathode active material production method disclosed herein While described, it is not intended to limit the invention to such specific embodiments.

≪Aqueous solution aq _A ≫
As shown in FIG. 3, the positive electrode active material manufacturing method according to the present embodiment prepares an aqueous solution (typically acidic, ie, an aqueous solution having a pH of less than 7) aq _A containing M _T (here, Ni, Co and Mn). (Step S110). In a preferred embodiment, the aqueous solution aq _A is a composition containing substantially no W. The amount ratio of each metal element contained in the aqueous solution aq _A can be appropriately set according to the composition of the positive electrode active material that is the object. For example, the molar ratio of Ni, Co, and Mn can be approximately the same as the molar ratio of these elements in the positive electrode active material. Incidentally, the aqueous solution aq _A may be one kind of the aqueous solution containing all M _T, or a two or more kinds of aqueous solutions having different compositions. For example, an aqueous solution _{aq A1} containing only Ni as _{M T,} two kinds of an aqueous solution _{aq A2} containing only Co and Mn as _{M T,} may be used as the aq _A. As one preferred embodiment, the mode of using one kind of an aqueous solution containing all of M _T are exemplified. Such an embodiment can be preferably employed from the viewpoints of avoiding complication of the manufacturing apparatus and easy control of manufacturing conditions.

The aqueous solution aq _A can be prepared, for example, by dissolving a predetermined amount of each of an appropriate Ni compound, Co compound, and Mn compound in an aqueous solvent. As these metal compounds, salts of each metal (namely, Ni salt, Co salt and Mn salt) can be preferably used. The order in which these metal salts are added to the aqueous solvent is not particularly limited. Alternatively, the aqueous solution aq _A may be prepared by mixing some or all of the metals in the form of an aqueous solution. For example, an aqueous solution containing a Mn salt may be mixed with an aqueous solution containing a Ni salt and a Co salt. The anions in these metal salts may be selected so that the salts have the desired water solubility. For example, it may be sulfate ion, nitrate ion, chloride ion, carbonate ion, hydroxide ion, and the like. All or part of these metal salt anions may be the same or different from each other. FIG. 3 shows an example in which the sulfate of each metal is used. The concentration of the aqueous solution aq _A can be, for example, a concentration at which the total of M _T (here, Ni, Co, Mn) is about 1.0 to 2.2 mol / L.

≪Aqueous solution aq _B ≫
The positive electrode active material manufacturing method of this aspect also includes preparing an aqueous solution aq _B containing W (step S120). The aqueous solution aq _B is typically substantially free of M _T (say that does not contain at least intentionally these metal elements, be incorporated as inevitable impurities may be tolerated.) It is a composition. For example, an aqueous solution aq _B containing substantially only W as a metal element can be preferably used. The aqueous solution aq _B can be prepared by dissolving a predetermined amount of W compound in an aqueous solvent in the same manner as the aqueous solution aq _A described above. Various W salts can be used as the W compound. In a preferred embodiment, a salt of tungstic acid (an oxo acid having W as a central element) is used. The W salt may be a solvate such as a hydrate. The cation in the W salt can be selected such that the salt is water-soluble, and can be, for example, ammonium ion, sodium ion, potassium ion, or the like. An example of a W salt that can be preferably used is ammonium paratungstate (5 (NH ₄ ) ₂ O · 12WO ₃ ). The concentration of W in the aqueous solution aq _B can be, for example, about 0.01 to 1 mol / L on the basis of W element.

The aqueous solvent used for the preparation of the aqueous solutions aq _A and aq _B is typically water, and may be a mixed solvent containing water as a main component. As the solvent other than water constituting the mixed solvent, an organic solvent (such as a lower alcohol) that can be uniformly mixed with water is preferable. In a preferred embodiment, water is used as the aqueous solvent. Depending on the solubility of each salt metal compound to be used, an aqueous solvent containing a reagent (acid, base, etc.) that improves solubility may be used.

≪Alkaline aqueous solution≫
The positive electrode active material manufacturing method disclosed herein, an aqueous solution aq _A containing _{M T,} an aqueous solution aq _B containing W, prepared as separate aqueous solutions, and alkaline conditions and aq _A and aq _B ( That is, mixing is performed under a condition that the pH exceeds 7. The hydroxide containing M _T and W from the mixed solution (hereinafter sometimes referred to as "transition metal hydroxide" or "precursor hydroxides".) Precipitating the (hydroxide formation step).

In order to achieve such alkaline conditions, the method according to this embodiment may comprise providing an alkaline aqueous solution aq _C. This alkaline aqueous solution aq _C is typically an aqueous solution in which an alkaline agent (a compound having an action of tilting the liquid property toward the alkali side) is dissolved in an aqueous solvent. As the alkali agent, any of strong bases (for example, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide) and weak bases (ammonia, amine, etc.) can be used. It is preferable to use an alkaline aqueous solution aq _C containing at least ammonia. In a preferred embodiment, an alkaline aqueous solution aq _C containing both weak and strong bases is used. For example, an alkaline aqueous solution aq _C containing ammonia and sodium hydroxide can be preferably used. A plurality of alkaline aqueous solutions having different compositions (for example, two types of aqueous ammonia and aqueous sodium hydroxide) are mixed in advance (step S125) or used as independent solutions (typically in a reaction vessel). Supply).

≪Nucleation stage≫
In a preferred embodiment, the hydroxide generation step comprises the steps of precipitating transition metal hydroxide nuclei from the mixed solution containing aq _A and aq _B (nucleation step), and growing the nuclei ( Particle growth stage). In a preferred embodiment, both the nucleation step and the particle growth step are performed in the presence of ammonia. It is particularly preferable that at least the particle growth stage is performed while controlling the ammonia concentration in the solution (for example, controlling it to a predetermined value or less). The particle growth stage is preferably carried out under alkaline conditions at a pH lower than that in the nucleation stage.

In the nucleation step (step S130), for example, an alkaline aqueous solution having an initial pH of about 11 to 14 (typically about 11.5 to 12.5, for example, around 12.0) is prepared in a reaction tank. While maintaining the initial pH, the aqueous solution aq _A and the aqueous solution aq _B can be preferably supplied to the reaction vessel at an appropriate rate and stirred and mixed. At this time, in order to maintain the initial pH, an alkaline aqueous solution may be additionally supplied to the reaction vessel as necessary.

  From the viewpoint of easily obtaining hollow active material particles with little variation (for example, a small number ratio of particles whose particle size and particle structure greatly deviate from the average), in the nucleation stage, a short time is required from the mixed solution. It is preferable to deposit a large number of nuclei within (for example, almost simultaneously). For example, the nuclei may be precipitated from a solution in which the transition metal hydroxide is in a supersaturated state (for example, by allowing the solution to reach a critical degree of supersaturation). In order to suitably realize such a precipitation mode, the nucleation step is performed under the condition of pH 12.0 or more (typically pH 12.0 or more and 14.0 or less, for example, pH 12.2 or more and 13.0 or less). Is advantageous.

In an embodiment in which an alkaline aqueous solution containing ammonia is used in the nucleation stage, the ammonia concentration in the reaction solution is not particularly limited, but it is usually suitably about 25 g / L or less, for example, 3 to 25 g / L (preferably 10 to 25 g / L). The pH and ammonia concentration can be adjusted by appropriately balancing the amount of ammonia water used and the amount of other alkaline agent used. The amount used can be grasped as a supply rate to the reaction system, for example. As the “other alkaline agent”, for example, a strong base such as sodium hydroxide or potassium hydroxide can be typically used in the form of an aqueous solution. In a preferred embodiment, sodium hydroxide is used as the other alkaline agent.
In addition, in this specification, the value of pH shall mean pH value on the basis of liquid temperature of 25 degreeC. Further, the ammonia concentration of the reaction solution can be measured by, for example, an ion chromatography method, an ion electrode method, or the like. For the measurement, a commercially available ion chromatograph device, an electrode type ammonia meter, or the like can be used.

≪Particle growth stage≫
In the particle growth stage (step S140), the transition metal hydroxide nuclei (typically particles) precipitated in the nucleation stage are preferably grown under alkaline conditions in a lower pH range than the nucleation stage. Let me. For example, the particles may be grown at a pH of less than 12.0 (typically pH 10.0 to less than 12.0, preferably pH 10.0 to 11.8, for example, pH 11.0 to 11.8). The transition metal hydroxide particles (precursor hydroxide particles) obtained through this particle growth stage typically have a structure in which the internal density of the particles is lower than the density of the outer surface portion of the particles. Have In an embodiment using an alkaline aqueous solution containing ammonia, in order to stably obtain the transition metal hydroxide particles having such a structure, it is important not to make the ammonia concentration in the liquid too high (keep it low) in the particle growth stage. is there. By this, the precipitation rate of the said transition metal hydroxide becomes quick, and the precursor hydroxide suitable for formation of the active material particle of a hollow structure (preferably perforated hollow structure) can be produced | generated effectively. Incidentally, the deposition rate of the transition metal hydroxide, for example, the to the total molar number of the metal ions supplied to the reaction solution (e.g., the M _T and W ions) contained in the reaction liquid of the liquid phase It can be grasped by examining the transition of the total number of moles of metal ions (total ion concentration).

  The ammonia concentration in the particle growth stage can be, for example, 25 g / L or less, and is usually suitably 20 g / L or less, preferably 15 g / L or less, more preferably 10 g / L or less (for example, 8 g). / L or less). The lower limit of the ammonia concentration in the liquid is not particularly limited, but from the viewpoints of ease of management of production conditions, quality stability, productivity, mechanical strength (for example, hardness) of the obtained active material particles, etc., the ammonia concentration is usually Is approximately 1 g / L or more (preferably 3 g / L or more). The pH and ammonia concentration in the particle growth stage can be adjusted in the same manner as in the nucleation stage. The ammonia concentration in the liquid in the particle growth stage may be approximately the same as the ammonia concentration in the nucleation stage, or may be lower than the ammonia concentration in the nucleation stage. In a preferred embodiment, the ammonia concentration in the liquid during the particle growth stage is controlled to be lower (typically 75% or less, for example, 50% or less) than the ammonia concentration in the nucleation stage.

In carrying out the technique disclosed herein, a precursor hydroxide suitable for the formation of an active material having a hollow structure (preferably a porous structure having pores) is obtained by suppressing the ammonia concentration in the particle growth stage as described above. Although it is not necessary to clarify the reason for obtaining, for example, the following can be considered. That is, for example, the following equilibrium reaction occurs in the mixed solution (reaction solution). M ₁ in the following formulas 1 and 2 is a transition metal (for example, Ni).

Here, when the ammonia concentration in the reaction solution is lowered, the equilibrium of Formula 1 shifts to the left and the concentration of M ₁ ²⁺ increases, so the balance of Formula 2 shifts to the right and M ₁ (OH) ₂ is generated. Is promoted. In other words, M ₁ (OH) ₂ is likely to precipitate. In such a situation where M ₁ (OH) ₂ is likely to precipitate, M ₁ (OH) ₂ is mainly precipitated because of the transition metal hydroxide already precipitated (nuclei generated in the nucleation stage, or M ₁ (OH) ₂ that occurs in the vicinity of the outer surface of the transition metal hydroxide particles in the course of the grain growth stage and precipitates inside the precipitate is reduced. As a result, precursor hydroxide particles having a structure in which the internal density is lower than the density of the outer surface portion (transition metal hydroxide particles suitable for forming hollow structure active material particles, the active material particles It is presumed that the precursor particles are formed as well.

Precursor hydroxide particles having such a structure are sintered so that, when mixed with a lithium compound and fired, the inside of the particles having a low density is taken into the vicinity of the outer surface where the density is high and the mechanical strength is high. To do. For this reason, as shown in FIG. 1, the shell 115 of the positive electrode active material particle 110 is formed, and a large hollow portion 116 is formed. Furthermore, when a crystal grows during sintering, a through hole 118 penetrating the shell 115 is formed in a part of the shell 115. Thereby, it is considered that the positive electrode active material particles 110 having the shell portion 115, the hollow portion 116, and the through hole 118 are formed. On the other hand, if the ammonia concentration in the particle growth stage is too high, the precipitation rate of M ₁ (OH) ₂ becomes small, so the difference between the precipitation amount in the vicinity of the outer surface and the precipitation amount in the inside becomes small, and the precursor of the above structure It is presumed that hydroxide particles tend to be difficult to form.

Moreover, in this way, an aqueous solution aq _A containing M _T, an aqueous solution aq _B containing W, prepared as a separate solution, precursor hydroxide particles that were produced by mixing them in alkaline conditions By mixing with a lithium compound and firing, a positive electrode active material in which W is present on the surface of primary particles can be effectively formed. This is because the aqueous solution aq _A comprising M _T is mixed with W after being neutralized, first hydroxide containing M _T begins to precipitate, easily W is precipitated by contact with the precipitate It is thought to be. Such precursor hydroxide particles are suitable for effectively suppressing crystal growth in the c-axis direction during firing and adjusting the full width at half maximum (A / B) to the above-described preferable range.

In each of the nucleation stage and the particle growth stage, the temperature of the reaction solution is controlled to a substantially constant temperature (eg, a predetermined temperature ± 1 ° C.) in a range of about 20 ° C. to 60 ° C. (eg, 30 ° C. to 50 ° C.). It is preferable. The temperature of the reaction solution may be approximately the same in the nucleation stage and the particle growth stage. The atmosphere in the reaction solution and the reaction vessel is preferably maintained in a non-oxidizing atmosphere (for example, a non-oxidizing atmosphere having an oxygen concentration of approximately 20% or less, preferably 10% or less) through the nucleation stage and the particle growth stage. . _{M T} ions contained in the reaction solution of the unit volume, the total number of moles of _{E A} ion and _{E B} ions (total ion concentration), through nucleation stage and the particle growth stage, for example, about 0.5 to 2.5 mol / L, usually about 1.0 to 2.2 mol / L is preferable. The aqueous solutions aq _A and aq _B may be replenished (typically continuously supplied) in accordance with the deposition rate of the transition metal hydroxide so that the concentration is maintained.

  The number of moles (ion concentration) of W ions contained in the reaction solution can be set to, for example, about 0.01 to 1.0 mol / L through the nucleation stage and the particle growth stage. In order to maintain such an ion concentration, the transition metal solution may be replenished (typically continuously supplied) in accordance with the deposition rate of the transition metal hydroxide. The amount of Ni ions, Co ions, Mn ions and W ions contained in the reaction solution is determined by the composition of the active material particles as the target (that is, the molar ratio of Ni, Co, Mn and W contained in the active material particles). It is preferable to make the ratio substantially corresponding.

  The time for continuing the precipitation reaction of the precursor hydroxide can be appropriately set according to the particle diameter (typically average particle diameter) of the target positive electrode active material. As a tendency, in order to obtain a positive electrode active material having a larger particle diameter, it is preferable to increase the reaction time. The transition metal hydroxide particles thus produced (here, composite hydroxide particles containing Ni, Co, Mn and W) are separated from the reaction solution after crystallization, and washed (typically It is preferable to prepare the particles having a desired particle size by washing with water and drying (step S150).

≪Mixing process≫
The positive electrode active material manufacturing method according to this aspect may include a mixing step of mixing the precursor hydroxide and a lithium compound (lithium source) (step S160). As the lithium compound, an oxide containing lithium may be used, and a compound that can be converted into an oxide by heating (lithium carbonate, nitrate, sulfate, oxalate, hydroxide, ammonium salt, sodium salt, etc.) May be used. Examples of preferable lithium compounds include lithium carbonate and lithium hydroxide. The said lithium compound can be used individually by 1 type or in combination of 2 or more types. In the positive electrode active material manufacturing method according to a preferred embodiment, lithium carbonate (for example, alone) is used as the lithium compound. In such an embodiment, it is particularly meaningful to adjust the Li—CO ₃ compound content of the positive electrode active material (reducing it to a predetermined value or less) by applying the production method disclosed herein. The mixing ratio of the precursor hydroxide and the lithium compound can be determined so that a desired molar ratio is realized in the target positive electrode active material. For example, the precursor hydroxide and the lithium compound may be mixed so that the molar ratio of lithium to another metal element is approximately the same as the molar ratio in the positive electrode active material.

≪Baking process≫
Then, the mixture is fired to produce a lithium transition metal oxide (step S170). This firing process includes the first firing stage (step S172) and the second firing stage (S174) described above. Preferably, after the firing step, the fired product is crushed and sieved as necessary to adjust the particle size of the positive electrode active material.

  Such a method for producing a positive electrode active material can be preferably applied particularly when the transition metal solution contains Ni. When the transition metal solution contains Ni, transition metal hydroxide particles are generated in the form of secondary particles in which a plurality of fine primary particles are aggregated when the transition metal hydroxide is precipitated from the solution. Further, in the temperature range at the time of firing, crystals grow while substantially maintaining the shape of the primary particles of the transition metal hydroxide.

<Particle porosity>
The positive electrode active material in the technology disclosed herein has a hollow structure (typically a holed hollow structure) having a particle porosity of 5% or more. A positive electrode active material having a particle porosity of 10% or more is preferable, and more preferably 15% or more. If the particle porosity is too small, the advantage of having a hollow structure may not be sufficiently exhibited. The particle porosity may be 20% or more (typically 23% or more, preferably 30% or more). The upper limit of the particle porosity is not particularly limited, but the durability of the active material particles (for example, the ability to withstand the compressive stress that can be applied during battery production or use and maintain the hollow shape), the ease of production, etc. Therefore, it is usually appropriate to make it 95% or less (typically 90% or less, for example 80% or less). In any one of the positive electrode active material manufacturing methods disclosed herein, the particle porosity is adjusted, for example, by a time during which the particle growth process is continued, a precipitation rate of transition metal hydroxide in the particle growth process (for example, ammonia Density) and the like.

Here, the “particle porosity” refers to the ratio of the hollow portion in the apparent cross-sectional area of the active material in the average of the cross sections obtained by cutting the positive electrode active material at random positions. This ratio can be grasped | ascertained through the scanning electron microscope (SEM) image in the suitable cross section of the positive electrode active material particle or the material containing this active material particle, for example. Such a cross-sectional SEM image is obtained, for example, by cutting a sample obtained by solidifying positive electrode active material particles or a material containing the active material particles with an appropriate resin (preferably a thermosetting resin), and observing the cross-section by SEM. Can do. In the cross-sectional SEM image, the shell part, the hollow part, and the through-hole of the active material particle can be distinguished by the difference in color tone or shade. The plurality of active material particles appear in any cross-sectional SEM image of the sample, and the area C _V hollow portion of their active material grains account, the cross-sectional area C _T occupied on their active material particles apparently A ratio (C _V / C _T ) is obtained. Here, the cross-sectional area C _T occupied on the active material particles apparently refers shell portion of the active material particles, the cross-sectional area occupied by the hollow portion and the through-hole. By this ratio (C _V / C _T ), the proportion of the active material particles in the apparent volume occupied by the hollow portions (that is, the particle porosity) is generally obtained.

Preferably, the value of the ratio (C _V / C _T ) is arithmetically averaged for an arbitrary plurality of cross-sectional SEM images of the sample. As the number of cross-sectional SEM images for determining the ratio (C _V / C _T ) increases in this way, and as the number of active material particles on which the ratio (C _V / C _T ) is calculated increases, the ratio ( The arithmetic mean value of C _V / C _T converges. Usually, it is preferable to determine the particle porosity based on at least 10 (for example, 20 or more) active material particles. Moreover, it is preferable to obtain | require particle | grain porosity based on the SEM image in the cross section of the arbitrary 3 places (for example, 5 places or more) of a sample at least.

≪Shell thickness≫
In such a positive electrode active material (positive electrode active material particles) having a hollow structure, the thickness of the shell (portion where primary particles are gathered in a spherical shell) is usually preferably 3.0 μm or less, preferably 2. It is 5 μm or less (for example, 2.2 μm or less), more preferably 2.0 μm or less (for example, 1.5 μm or less). The smaller the thickness of the shell portion, the more easily lithium ions are released from the inside of the shell portion (center portion of the thickness) during charging, and the lithium ions are easily absorbed up to the inside of the shell portion during discharging. Therefore, it is possible to increase the amount of lithium ions that can be occluded and released by the active material particles of a unit mass under a predetermined condition, and to reduce resistance when the active material particles occlude and release lithium ions. A lithium secondary battery using such active material particles is excellent in output in a low SOC range in combination with the effect that the half-width ratio (A / B) is in a preferable range disclosed herein. Can be.

  Although the lower limit of the thickness of the shell is not particularly limited, it is generally preferable that the thickness is approximately 0.1 μm or more. By making the thickness of the shell portion 0.1 μm or more, it is possible to ensure higher durability against stress that can be applied at the time of manufacturing or using the battery, expansion / contraction of the positive electrode active material accompanying charge / discharge, etc. Can do. Thereby, the performance of the lithium secondary battery can be stabilized. From the viewpoint of achieving both an internal resistance reduction effect and durability, the thickness of the shell is preferably about 0.1 μm to 2.2 μm, more preferably 0.2 μm to 1.5 μm, and particularly preferably Is 0.5 μm to 1 μm.

  Here, the thickness of the shell portion 115 (see FIG. 1) refers to the inner side surface 115a (however, the portion corresponding to the through hole 118) of the shell portion 115 in the cross-sectional SEM image of the positive electrode active material or the material containing the active material particles. Is not included in the inner side surface 115a.) The average value of the shortest distance T (k) from any position k to the outer side surface 115b of the shell 115. More specifically, the shortest distance T (k) may be obtained for a plurality of positions on the inner surface 115a of the shell 115, and the arithmetic average value thereof may be calculated. In this case, as the number of points for obtaining the shortest distance T (k) is increased, the thickness T of the shell 115 converges to the average value, and the thickness of the shell 115 can be appropriately evaluated. Usually, it is preferable to determine the thickness of the shell 115 based on at least 10 (for example, 20 or more) active material particles 110. In addition, it is preferable to obtain the thickness of the shell 115 based on SEM images in at least three (for example, five or more) cross sections of the sample (for example, positive electrode active material).

≪Secondary particle shape≫
The average particle diameter (secondary particle diameter) of the active material particles is preferably about 2 μm to 25 μm, for example. According to the positive electrode active material having such a configuration, good battery performance can be more stably exhibited. If the average particle size is too small, the volume of the hollow portion is small, and the effect of improving battery performance may tend to be reduced. More preferably, the average particle size is about 3 μm or more. From the viewpoint of productivity of the positive electrode active material, the average particle size is preferably about 25 μm or less, more preferably about 15 μm or less (for example, about 10 μm or less). In a preferred embodiment, the average particle diameter of the positive electrode active material is about 3 μm to 10 μm (for example, 3 μm to 8 μm).

  The average particle diameter of the active material particles can be obtained as a volume-based median diameter (D50) measured by a method known in the art, for example, a laser diffraction scattering method. The average particle diameter can be adjusted, for example, through the pH in the nucleation stage, the time during which the particle growth process is continued, the precipitation rate of the transition metal hydroxide in the particle growth process, and the like. The deposition rate of the transition metal hydroxide can be adjusted, for example, through the concentration, pH, reaction system temperature, and the like of one or more chemical species related to the above formula 1 or formula 2. Alternatively, the average particle diameter may be adjusted by selecting particles by general sieving. Such an average particle diameter adjusting method can be carried out alone or in appropriate combination.

≪Primary particle shape≫
The primary particles 112 constituting the active material particles 110 may have a major axis L1 of, for example, approximately 0.1 μm to 1.0 μm. According to the knowledge of the present inventor, the major axis L1 of the primary particles 112 can be roughly correlated with the crystal size in the direction orthogonal to the normal direction (c-axis) of the (003) plane. If L1 is too small, the capacity maintenance of the battery may tend to decrease. From this viewpoint, a positive electrode active material having L1 of 0.2 μm or more is preferable, more preferably 0.3 μm or more, and still more preferably 0.4 μm or more. On the other hand, if L1 is too large, the distance (Li ion diffusion distance) from the surface of the crystal to the inside (the center portion of L1) becomes long, so that the ion diffusion into the crystal becomes slow, and the output characteristics (particularly low The output characteristics in the SOC region tend to be low. From this viewpoint, L1 is preferably 0.8 μm or less (for example, 0.75 μm or less). In a preferred embodiment, the major particle L1 of the primary particles is 0.2 μm to 0.8 μm (for example, 0.3 μm to 0.75 μm).

  The major axis L1 of the primary particle 112 can be measured based on, for example, an SEM image of the particle surface of the positive electrode active material particle (secondary particle) 110. When measuring the primary particle size of the positive electrode active material particles contained in the positive electrode active material layer of the lithium secondary battery, the SEM on the surface of the positive electrode active material particles appearing in the cross section obtained by dividing the active material layer is used. It is good to observe. For example, in the SEM image, primary particles 112 suitable for specifying the long diameter L1 are specified. That is, since a plurality of primary particles 112 are shown in the SEM image of the particle surface of the positive electrode active material particles (secondary particles) 110, a plurality of these primary particles 112 are extracted in descending order of the display area in the SEM image. To do. Thereby, in the SEM image of the particle surface, it is possible to extract the primary particles 112 in which the outer shape along the longest major axis L1 is reflected. And it is good to make the length of the longest long axis in the extracted primary particle 112 into the major axis L1.

≪Specific surface area≫
The BET specific surface area of the positive electrode active material disclosed herein is preferably about 0.3 m ² / g or more, more preferably 0.5 m ² / g or more, and further preferably 0.8 m ² / g or more. is there. Further, the BET specific surface area of the active material particles 110 can be, for example, about 3.0 m ² / g or less (for example, 2.0 m ² / g or less), and may be 1.7 m ² / g or less. Further, it may be 1.5 m ² / g or less. The positive electrode active material according to a preferred embodiment has a BET specific surface area of approximately 0.5 to 2.0 m ² / g.

≪Positive electrode active material hardness≫
According to a preferred embodiment of the technology disclosed herein, positive electrode active material particles having an average hardness of approximately 0.5 MPa or more (typically 1.0 MPa or more, for example, 2.0 to 10 MPa) can be produced. Here, the “average hardness” refers to a value obtained by dynamic microhardness measurement performed using a flat diamond indenter with a diameter of 50 μm and under a load speed of 0.5 mN / sec to 3 mN / sec. For the dynamic microhardness measurement, for example, a microhardness meter manufactured by Shimadzu Corporation, model “MCT-W500” can be used. As the hardness measurement is performed for more active material particles, the arithmetic average value of the hardness of those active materials converges. As the average hardness, it is preferable to employ an arithmetic average value based on at least three (preferably five or more) active material particles. The above-described method for producing perforated active material particles including a nucleation step and a particle growth step is suitable as a method for producing perforated active material particles having such average hardness. The active material particles having a hollow structure obtained by this method are, for example, compared to the positive electrode active material particles having a general porous structure obtained by, for example, a spray baking method (also referred to as a spray-drying method). It can be harder (higher average hardness) and excellent in form stability. Thus, the active material particles having a hollow structure and high average hardness (in other words, high shape maintaining ability) can give a battery that stably exhibits higher performance.

  According to the present invention, a positive electrode having any of the positive electrode active materials disclosed herein is provided. In addition, a lithium secondary battery (typically a lithium ion secondary battery) including the positive electrode is provided. Hereinafter, an embodiment of such a lithium secondary battery will be described in detail by taking, as an example, a lithium ion secondary battery having a configuration in which a wound electrode body and a nonaqueous electrolytic solution are housed in a flat rectangular battery case. Although described, it is not intended to limit embodiments of the technology disclosed herein.

≪Lithium ion secondary battery≫
In the lithium ion secondary battery according to an embodiment of the technology disclosed herein, for example, as shown in FIGS. 4 and 5, the wound electrode body 20 and the non-aqueous electrolyte 90 are used together with the electrode body 20. It has the structure accommodated in the flat box-shaped battery case 10 corresponding to a shape. The opening 12 of the case 10 is closed with a lid 14. The lid body 14 is provided with a positive terminal 38 and a negative terminal 48 for external connection so that a part of the terminals protrudes outward from the lid body 14. The lithium ion secondary battery 100 having such a configuration is provided in the lid body 14 after the electrode body 20 is accommodated inside the opening portion 12 of the case 10 and the lid body 14 is attached to the opening portion 12 of the case 10, for example. It can be constructed by injecting an electrolytic solution 90 from a prepared electrolytic solution injection hole (not shown) and then closing the injection hole.

  The electrode body 20 has a positive electrode sheet 30 in which a positive electrode active material layer 34 containing a positive electrode active material is held by a long sheet-like positive electrode current collector 32 and a negative electrode active material layer 44 containing a negative electrode active material in a long sheet form. The negative electrode sheet 40 held on the negative electrode current collector 42 is overlapped and wound, and the obtained wound body is pressed from the side surface direction and flared to form a flat shape. Typically, an insulating layer is disposed between the positive electrode active material layer 34 and the negative electrode active material layer 44 to prevent direct contact therebetween. In a preferred embodiment, two long sheet-like separators 50 are used as the insulating layer. For example, the electrode body 20 is configured by winding these separators 50 together with the positive electrode sheet 30 and the negative electrode sheet 40. The insulating layer may be coated on one or both surfaces of the positive electrode active material layer 34 and the negative electrode active material layer 44.

≪Positive electrode sheet≫
The positive electrode sheet 30 is formed such that the positive electrode active material layer 34 is not provided at one end along the longitudinal direction, and the positive electrode current collector 32 is exposed. Similarly, the negative electrode sheet 40 is formed so that the negative electrode active material layer 44 is not provided at one end along the longitudinal direction, and the negative electrode current collector 42 is exposed. A positive electrode terminal 38 is joined to the exposed end portion of the positive electrode current collector 32, and a negative electrode terminal 48 is joined to the exposed end portion of the negative electrode current collector 42. The positive and negative electrode terminals 38 and 48 and the positive and negative electrode current collectors 32 and 42 can be joined by, for example, ultrasonic welding, resistance welding, or the like.

  The positive electrode sheet 30 is a paste-like or slurry-like composition in which any of the positive electrode active materials disclosed herein is dispersed in an appropriate solvent together with a conductive material, a binder (binder) and the like used as necessary. (Dispersion for forming the positive electrode active material layer) is preferably applied to the positive electrode current collector 32 and dried to dry the composition. As the solvent, any of an aqueous solvent and an organic solvent can be used. From the viewpoint of highly preventing the W contained in the positive electrode active material from eluting into the solvent, it is preferable to use an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) as the solvent.

  As the conductive material, a conductive powder material such as carbon powder or carbon fiber is preferably used. As the carbon powder, various carbon blacks such as acetylene black, furnace black, ketjen black, and graphite powder are preferable. A conductive material can be used alone or in combination of two or more.

  Examples of the binder include carboxymethyl cellulose (CMC; typically sodium salt), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), and the like. It is done. Such a binder can be used individually by 1 type or in combination of 2 or more types as appropriate. Such a binder can also function as a thickener for the positive electrode active material composition.

  The proportion of the positive electrode active material in the entire positive electrode active material layer is suitably about 50% by mass or more (typically 50 to 95% by mass), and usually about 70 to 95% by mass. preferable. When the conductive material is used, the ratio of the conductive material in the entire positive electrode active material layer can be, for example, about 2 to 20% by mass, and is usually preferably about 2 to 15% by mass. When using a binder, the ratio of the binder to the whole positive electrode active material layer can be, for example, about 0.5 to 10% by mass, and usually about 1 to 5% by mass is appropriate.

  As the positive electrode current collector 32, a conductive member made of a highly conductive metal is preferably used. For example, aluminum or an alloy containing aluminum as a main component can be used. The shape of the positive electrode current collector 32 may vary depending on the shape of the lithium ion secondary battery, and is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape. . In the lithium ion secondary battery 100 including the wound electrode body 20 as in the present embodiment, an aluminum sheet (aluminum foil) having a thickness of about 10 μm to 30 μm can be preferably used as the positive electrode current collector 32.

The dispersion applied to the positive electrode current collector 32 can be dried under heating as necessary. After drying, the whole may be pressed as necessary. The mass of the positive electrode active material layer 34 provided per unit area of the positive electrode current collector 32 (the total mass of both surfaces in the configuration having the positive electrode active material layer 34 on both surfaces of the positive electrode current collector 32) is, for example, 5 to 40 mg / cm. ² (typically 5 to 20 mg / cm ² ) is appropriate. The density of the positive electrode active material layer 34 can be, for example, about 1.0 to 3.0 g / cm ³ (typically 1.5 to 3.0 g / cm ³ ).

≪Negative electrode sheet≫
The negative electrode sheet 40 is, for example, a negative electrode current collector made of a paste or slurry-like composition (dispersion for forming a negative electrode active material layer) in which a negative electrode active material is dispersed in an appropriate solvent together with a binder used as necessary. It can preferably be prepared by applying to the body 42 and drying the composition.

  As the negative electrode active material, one type or two or more types of materials conventionally used for lithium ion secondary batteries can be used without any particular limitation. A carbon material is mentioned as a suitable negative electrode active material. Particulate carbon materials (carbon particles) having a graphite structure (layered structure) at least partially are preferable. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), easily graphitized carbon material (soft carbon), or a combination of these materials is preferably used. obtain. Among these, graphite particles such as natural graphite can be preferably used. Carbon particles in which amorphous (amorphous) carbon is added to the surface of graphite may be used. Such carbon particles can be a negative electrode active material suitable for more rapid charge / discharge (for example, high output discharge). The proportion of the negative electrode active material in the entire negative electrode active material layer is not particularly limited, but it is usually appropriately about 50% by mass or more, preferably about 90 to 99% by mass (for example, about 95 to 99% by mass). It is.

  As a binder, the thing similar to the positive electrode mentioned above can be used individually or in combination of 2 or more types. What is necessary is just to select the addition amount of a binder suitably according to the kind and quantity of a negative electrode active material, for example, can be about 1-5 mass% of the whole negative electrode active material layer.

  As the negative electrode current collector 42, a conductive member made of a metal having good conductivity is preferably used. For example, copper or an alloy containing copper as a main component can be used. Further, the shape of the negative electrode current collector 42 can be in various forms like the positive electrode current collector 32. In the lithium ion secondary battery 100 including the wound electrode body 20 as in the present embodiment, a copper sheet (copper foil) having a thickness of about 5 μm to 30 μm can be preferably used as the negative electrode current collector 42.

The dispersion applied to the negative electrode current collector 42 can be dried under heating as necessary. After drying, the whole may be pressed as necessary. The mass (total mass of both surfaces) of the negative electrode active material layer 44 provided per unit area of the negative electrode current collector 42 is, for example, about 3 to 30 mg / cm ² (typically 3 to 15 mg / cm ² ). Is appropriate. The density of the negative electrode active material layer 44 can be, for example, about 0.8 to 2.0 g / cm ³ (typically 1.0 to 2.0 g / cm ³ ).

≪Capacitance ratio between positive electrode and negative electrode≫
Although not particularly limited, the ratio of the initial capacity (C _N ) of the negative electrode to the initial capacity (C _P ) of the positive electrode (C _N / C _P ) is usually 1.0 to 2.0, for example. It is suitable and it is preferable to set it as 1.2-1.9. If C _N / C _P is too small, inconveniences such as easy deposition of metallic lithium may occur depending on the use conditions of the battery (for example, during rapid charging). When C _N / C _P is too large, the energy density of the battery may be easily lowered.

≪Separator≫
As the separator 50 interposed between the positive electrode sheet 30 and the negative electrode sheet 40, the same separator as a general separator in the field can be used without particular limitation. For example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide, a nonwoven fabric, or the like can be used. Preferable examples include a single layer or multilayer structure porous sheet (microporous resin sheet) mainly composed of one or more kinds of polyolefin resins. For example, a PE sheet, a PP sheet, a sheet having a three-layer structure (PP / PE / PP structure) in which PP layers are laminated on both sides of the PE layer, and the like can be suitably used. The thickness of the separator is preferably set within a range of approximately 10 μm to 40 μm, for example. The separator in the technique disclosed herein may have a configuration in which a porous heat-resistant layer is provided on one side or both sides (typically, one side) of the porous sheet or the nonwoven fabric. Such a porous heat-resistant layer can be, for example, a layer containing an inorganic material (inorganic fillers such as alumina particles can be preferably employed) and a binder.

≪Electrolytic solution≫
As the nonaqueous electrolyte solution 90, a solution containing an electrolyte (supporting salt) in a nonaqueous solvent (organic solvent) is used. As said non-aqueous solvent, the organic solvent used for the electrolyte solution of a general lithium ion secondary battery can be used, selecting suitably 1 type, or 2 or more types. Examples of particularly preferred non-aqueous solvents include carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), vinylene carbonate (VC), and propylene carbonate (PC). The For example, a mixed solvent containing EC, EMC, and DMC at a volume ratio of 3: 3: 4 can be preferably used.

As the electrolyte, one or more lithium salts used as an electrolyte in a general lithium ion secondary battery can be appropriately selected and used. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiEBF ₆ , Li (CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO _{3 and the} like. A particularly preferred example is LiPF ₆ . The nonaqueous electrolytic solution 90 is preferably prepared so that, for example, the electrolyte concentration is within a range of 0.7 to 1.3 mol / L (typically 1.0 to 1.2 mol / L).

The non-aqueous electrolyte 90 may contain an optional additive as necessary as long as the object of the present invention is not significantly impaired. Such additives include one or more purposes such as, for example, improvement in output performance of the battery 100, improvement in storage stability (suppression of capacity reduction during storage, etc.), improvement in cycle characteristics, improvement in initial charge / discharge efficiency, and the like. Can be used in Examples of preferable additives include fluorophosphate (preferably difluorophosphate, for example, lithium difluorophosphate represented by LiPO ₂ F ₂ ), lithium bisoxalate borate (LiBOB), and the like. The concentration of each additive in the nonaqueous electrolytic solution 90 is usually suitably 0.20 mol / L or less (typically 0.005 to 0.20 mol / L), for example, 0.10 mol / L. Or less (typically 0.01 to 0.10 mol / L). As a preferable embodiment, a nonaqueous electrolytic solution 90 containing both LiPO ₂ F ₂ and LiBOB at a concentration of 0.01 to 0.05 mol / L (for example, 0.025 mol / L, respectively) can be given.

  The wound electrode body 20 is accommodated in the battery case 10, and an appropriate nonaqueous electrolytic solution 90 is placed (injected) into the case 10, and the wound electrode body 20 is soaked. Then, the construction (assembly) of the lithium secondary battery 100 according to the present embodiment is completed by sealing the opening 12 of the case 10 by welding with the lid 14 or the like. In addition, the sealing process of the battery case 10 and the arrangement | positioning (injection) process of the electrolyte solution 90 can be performed similarly to the method currently performed by manufacture of the conventional lithium secondary battery. Thereafter, the battery is conditioned (initial charge / discharge). You may perform processes, such as degassing and a quality inspection, as needed.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

<< Manufacture of active material particles having a hollow structure >>
(Sample P1)
Nickel sulfate (NiSO ₄ ), cobalt sulfate (CoSO ₄ ) and manganese sulfate (MnSO ₄ ) are dissolved in water, the molar ratio of Ni: Co: Mn is 1: 1: 1, and Ni, Co and Mn An aqueous solution aq _A having a total concentration of 1.8 mol / L was prepared. Also, ammonium paratungstate (5 (NH ₄ ) ₂ O · 12WO ₃ ) was dissolved in water to prepare an aqueous solution aq _B (W aqueous solution) having a W concentration of 0.05 mol / L.

About half the volume of water was placed in a reaction vessel equipped with a stirrer and a nitrogen introduction tube, and heated to 40 ° C. with stirring. After the atmosphere in the reaction vessel was replaced with nitrogen, 25% (mass basis) sodium hydroxide aqueous solution and 25% (mass basis) were maintained in a non-oxidizing atmosphere with an oxygen concentration of 2.0% under a nitrogen stream. Standard) Aqueous aqueous solution (NH ₃ · NaOH aqueous solution) having a pH of 12.0 based on a liquid temperature of 25 ° C. and an ammonia concentration of liquid phase of 20 g / L was prepared by adding appropriate amounts of ammonia water. .

By supplying the aqueous solution aq _A , the aqueous solution aq _B , the aqueous solution aq _B , the 25% aqueous sodium hydroxide solution, and the 25% aqueous ammonia prepared at the above rate to the alkaline aqueous solution in the reaction vessel, respectively, Hydroxides were crystallized from the reaction solution while maintaining the pH at 12.0 or higher (specifically, pH 12.0 to 14.0) and an ammonia concentration of 20 g / L (nucleation stage).

  Next, the supply rate of each solution to the reaction vessel is adjusted to adjust the reaction solution to a pH of less than 12.0 (specifically, pH 10.5 to 11.9, and the liquid phase ammonia concentration is 1 to 10 g / The particle growth reaction of the nuclei generated above was performed while controlling to a predetermined concentration in the range of L. (Particle growth stage) The product was taken out of the reaction vessel, washed with water, dried and (Ni + Co + Mn): W A composite hydroxide (precursor hydroxide) having a molar ratio of 100: 0.5 was obtained, and the precursor hydroxide was subjected to a heat treatment at 150 ° C. for 12 hours in an air atmosphere.

Thereafter, the precursor hydroxide and Li ₂ CO ₃ (lithium source) were mixed so that the molar ratio of (Ni + Co + Mn): Li (ie, m _T : m _Li ) was 1.16: 1 (mixing) Process). This unfired mixture is subjected to a first firing stage in which the maximum firing temperature is maintained at 900 ° C. (T1) for 3 hours (t1) in an air atmosphere, and then the maximum firing temperature is maintained at 730 ° C. (T2) for 8 hours (t2). Subjected to the second firing stage. Thereafter, the fired product was cooled, crushed and sieved. In this way, the positive electrode active material sample P1 having an average composition represented by Li _1.16 Ni _1/3 Co _1/3 Mn _1/3 W _0.005 O ₂ and having the particle shape shown in Table 1 Got.

(Samples P2 to P4)
Table 1 shows the maximum firing temperature (T1) and holding time (t1) in the first firing stage, the maximum firing temperature (T2) and holding time (t2) in the second firing stage, and m _T : m _Li in the mixing step. Except for the above settings, positive electrode active material samples P2 to P4 were obtained in the same manner as in the manufacture of sample P1.
FIG. 6 shows a temperature profile (firing pattern I) of the firing process related to the manufacture of samples P1 to P4.

(Samples P5 to P8)
Except for not performing the second firing step, positive electrode active material samples P5 to P8 corresponding to the respective P1 to P4 were obtained in the same manner as the manufacture of the samples P1 to P4.
FIG. 7 shows a temperature profile (firing pattern II) of the firing process for manufacturing samples P5 to P8.

(Samples P9 to P11)
Production of sample P1 except that m _T : m _Li in the mixing step, maximum firing temperature (T1) and holding time (t1) are set as shown in Table 1, and the second firing step is not performed. Similarly, positive electrode active material samples P9 to P11 were obtained.
FIG. 8 shows a temperature profile (firing pattern III) of the firing process related to the manufacture of samples P9 to P11.

In addition, all of the positive electrode active material samples P1 to P11 were prepared such that the average particle diameter (median diameter D50) was approximately 3 to 8 μm. When the cross-sectional SEM observation of these positive electrode active material samples P1-P11 was performed, all were the form of the secondary particle which the primary particle gathered, Comprising: The clear shell part and the hollow part were provided. It was confirmed that several (on average, one or more) through holes were formed in the shell, and that the shell was densely sintered at portions other than the through holes. Moreover, when the average hardness of sample P1-P11 was measured by the method mentioned above, it was confirmed that all exist in the range of 0.5 MPa-10 MPa. In addition, the BET specific surface areas of Samples P1 to P11 were all in the range of 0.5 to 2.0 m ² / g.

(Particle shape)
The outer shell thickness (shell thickness) and porosity (cross-sectional area ratio) of the positive electrode active material samples P1 to P11 were determined by the method described above.
(Half width ratio (A / B))
The powder X-ray diffraction patterns of Samples P1 to P11 are measured by the above method, and the half width A of the peak obtained by the diffractive surface with the Miller index (003) and the half width of the peak obtained by the diffractive surface with the Miller index (104). The ratio with B (A / B) was calculated.

(Li-CO ₃ compound content)
With respect to the positive electrode active material samples P1 to P11, neutralization titration of Li—CO ₃ compound was performed using an automatic titration apparatus (model “COM-1600” manufactured by Hiranuma Sangyo Co., Ltd.) according to the following procedure.
(1) A positive electrode active material sample S (g) (typically about 2.01 g to 2.04 g) is weighed to the order of 0.0001 g in a 200 mL beaker containing a stirrer.
(2) Add ion exchange water to the beaker until the volume reaches 100 mL.
(3) Add 2 mL of 10% barium chloride solution.
(4) Stir with a stirrer for about 1 minute.
(5) Using an automatic titration apparatus, neutralization titration is performed with a hydrochloric acid solution using the inflection point near pH 9 as the first neutralization point, and the hydrochloric acid titration amount D1 (L) up to the first neutralization point is obtained.
(6) Further titration is performed, neutralization titration is performed with a hydrochloric acid solution using the inflection point near pH 4 as the second neutralization point, and the hydrochloric acid titration amount D2 (L) up to the second neutralization point is obtained.
(7) The amount of Li—CO ₃ compound is calculated from the difference between D1 and D2.

The concentration of the hydrochloric acid solution used was 1 mol / L.
Other titration conditions were as follows.
Titration speed: 400 μL / sec Maximum drop volume: 50 μL
Minimum drop volume: 2μL
Drop waiting time: 1 second Stability wait coefficient: 10mpH
Stabilization wait time: 1 second

≪Performance evaluation≫
Test batteries (lithium ion secondary batteries) were constructed using the positive electrode active material samples P1 to P11, and their performance was evaluated.

(Construction of lithium ion secondary battery)
The active material particle sample obtained above, AB (conductive material), and PVDF (binder) are N-methylpyrrolidone (NMP) so that the mass ratio of these materials is 90: 8: 2. And a slurry-like composition for forming a positive electrode active material layer was prepared. A positive electrode sheet having a positive electrode active material layer on both sides of the positive electrode current collector by applying this composition to both sides of a long aluminum foil (positive electrode current collector) having a thickness of 15 μm and drying it. Was made. The coating amount of the composition for the positive electrode active material layer was adjusted so that the coating amount on both sides was substantially equal and the total coating amount on both sides was about 11.8 mg / cm ² (based on solid content). After drying, the density of the positive electrode active material layer was adjusted to about 2.3 g / cm ³ by pressing with a rolling press.

As the negative electrode active material, carbon particles having a structure in which amorphous carbon was coated on the surface of graphite particles were used. More specifically, natural graphite powder and pitch are mixed and the pitch is adhered to the surface of the graphite powder (the mass ratio of natural graphite powder: pitch is 96: 4), and 1000 under an inert atmosphere. After baking for 10 hours at a temperature of 1 ° C. to 1300 ° C., it was sieved to obtain a negative electrode active material having an average particle size (D50) of 8 to 11 μm and a specific surface area of 3.5 to 5.5 m ² / g. This negative electrode active material, SBR, and CMC are mixed with ion-exchanged water so that the mass ratio of these materials is 98.6: 0.7: 0.7, to form a slurry for forming the negative electrode active material layer A composition was prepared. A negative electrode sheet having a negative electrode active material layer on both sides of the negative electrode current collector was prepared by applying this composition to both sides of a long copper foil (negative electrode current collector) having a thickness of 10 μm and drying it. did. The coating amount of the negative electrode active material layer composition was adjusted so that the coating amount on both sides was substantially equal, and the total coating amount on both sides was about 7.5 mg / cm ² (based on solid content). Dried, and pressed by rolling pressing machine was adjusted density of the negative electrode active material layer to about ^{1.0g / cm 3 ~1.4g / cm 3} .

The positive electrode sheet and the negative electrode sheet are laminated and wound through two separators (single-layer structure made of porous polyethylene is used), and the wound body is pressed from the side direction to be ablated. Thus, a flat wound electrode body was produced. This wound electrode body was housed in a box-type battery container together with a non-aqueous electrolyte, and the opening of the battery container was hermetically sealed. As the non-aqueous electrolyte, a mixed solvent containing EC, EMC, and DMC at a volume ratio of 3: 3: 4 and containing LiPF ₆ as a supporting salt at a concentration of about 1 mol / L was used. . It should be noted that an electrolytic solution in which difluorophosphate (LiPO ₂ F ₂ ) and lithium bisoxalate borate (LiBOB) are arbitrarily dissolved at a rate of about 0.05 mol / L is used. You can also. A test battery was assembled in this manner. In this test battery, the initial capacity ratio (C _N / C _P ) described above is adjusted to 1.5 to 1.9.

(conditioning)
The test battery constructed as described above was conditioned according to the following procedures 1 to 3.
[Procedure 1] After reaching 3.0 V by 1 C constant current discharge, discharge at a constant voltage for 2 hours and rest for 10 seconds.
[Procedure 2] After reaching 4.1 V by constant current charging at 1 C, charging is performed for 2.5 hours by constant voltage charging, and rests for 10 seconds.
[Procedure 3] After reaching 3.0 V by constant current discharge of 0.5 C, discharge by constant voltage discharge for 2 hours and stop for 10 seconds.
The discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in the procedure 3 is defined as the rated capacity.

(Measurement of rated capacity (initial capacity))
About the test battery after the said conditioning, the rated capacity was measured according to the following procedures 1-3 in the temperature range of 25 degreeC and the voltage range of 3.0V to 4.1V.
[Procedure 1] Discharge to 3.0 V at a constant current of 1 C, then discharge at a constant voltage for 2 hours, and rest for 10 seconds.
[Procedure 2] Charge to 4.1 V with a constant current of 1 C, then charge with a constant voltage for 2.5 hours, and rest for 10 seconds.
[Procedure 3] Discharge to 3.0 V with a constant current of 0.5 C, then discharge with constant voltage for 2 hours, and stop for 10 seconds.
The discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in the procedure 3 was defined as the rated capacity (initial capacity). In this test battery, the rated capacity was about 4 Ah.

(Measurement of low-temperature and low-SOC output)
The output at −30 ° C. of a test battery adjusted to a low SOC (here, 27%) was measured by the following procedures 1 to 5. The results are shown in Tables 1 and 2.
[Procedure 1] The battery after the above conditioning process and rated capacity measurement is charged from 3 V to SOC 27% (CC charge) at a constant current of 1 C in a temperature environment of room temperature (here, 25 ° C.), and then at the same voltage. Charge for 2.5 hours (CV charge).
[Procedure 2] The battery after Procedure 1 is held in a thermostat at -30 ° C for 6 hours.
[Procedure 3] The battery after Procedure 2 is discharged at a constant watt (W) in a temperature environment of −30 ° C., and the number of seconds from the start of discharge until the voltage reaches 2.0 V (discharge cut voltage) is measured. .
[Procedure 4] The discharge output (discharge wattage of constant watt discharge) in the constant watt discharge of step 3 is varied between 80 W and 200 W (more specifically, the discharge output in the constant watt discharge of step 3 is The above steps 1 to 3 are repeated while increasing the first 80W, the second 90W, the third 100W,.
[Procedure 5] From the approximate curve of the plot in which the horizontal axis represents the number of seconds up to a voltage of 2.0 V measured in each constant watt discharge of procedure 4 and the vertical axis represents the constant watt discharge output at that time, the voltage 2. An output value (low temperature low SOC output) when the number of seconds up to 0 V is 2 seconds is obtained.

The measurement results of the low temperature and low SOC output are shown in Table 2 and FIG. In FIG. 9, the plot of group I is the measurement result of the test battery using the positive electrode active material samples P1 to P4 manufactured by the firing pattern (firing pattern I) shown in FIG. Group II plots are measurement results for test batteries using positive electrode active material samples P5 to P8 manufactured by the firing pattern (firing pattern II) shown in FIG. The plot of group III is a measurement result of the test battery using the positive electrode active material samples P9 to P11 manufactured by the firing pattern (firing pattern III) shown in FIG. In Table 2, the full width at half maximum (A / B) and the amount of Li—CO ₃ compound shown in Table 1 are listed again.

As shown in these charts, the positive electrode active material samples P1 to P4 to which the firing pattern I (FIG. 6) having the first firing stage and the second firing stage is applied are all (003) plane / (104). The half width ratio of the surface is (A / B) 0.4 to 0.7 (more specifically, 0.4 or more and less than 0.7), and the Li—CO ₃ compound content is 0.2 mass% or less. It was something to satisfy. According to the test batteries using these positive electrode active material samples P1 to P4, an excellent low-temperature and low-SOC output of 100 W or more was realized under the above evaluation conditions.

In contrast, among the positive electrode active material samples P5 to P11 to which the firing pattern having no second firing stage is applied, the firing time is set so that the half width ratio (A / B) satisfies 0.4 to 0.7. In the set samples P5 to P8, the content of the Li—CO ₃ compound greatly exceeded 0.2% by mass. Samples P8 to P11 in which the firing time was set so that the content of the Li—CO ₃ compound was 0.2% by mass or less had a half-width ratio (A / B) greatly exceeding 0.7. It was. And the low temperature low SOC output of the battery for evaluation using these samples P5-P11 was all clearly lower than the battery for evaluation using samples P1-P4.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

  Since the lithium secondary battery provided by the technology disclosed herein exhibits excellent performance as described above, it can be used as a lithium secondary battery for various applications. For example, it is suitable as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. Such lithium secondary batteries may be used in the form of an assembled battery formed by connecting a plurality of them in series and / or in parallel. Therefore, according to the technique disclosed herein, as schematically shown in FIG. 10, a vehicle (typically an automobile, in particular, a vehicle including such a lithium secondary battery (which may be in the form of an assembled battery) 100 as a power source. A vehicle including an electric motor such as a hybrid vehicle, an electric vehicle, and a fuel cell vehicle) 1 may be provided.

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Battery case 12 Opening part 14 Cover body 20 Winding electrode body 30 Positive electrode sheet (positive electrode)
32 Positive electrode current collector 34 Positive electrode active material layer 38 Positive electrode terminal 40 Negative electrode sheet (negative electrode)
42 Negative electrode current collector 44 Negative electrode active material layer 48 Negative electrode terminal 50 Separator 90 Nonaqueous electrolyte 100 Lithium ion secondary battery (lithium secondary battery)
110 Positive electrode active material particles (positive electrode active material)
112 Primary particle 115 Shell portion 115a Shell inner surface 115b Shell outer surface 116 Hollow portion 118 Through hole

Claims (6)

A lithium secondary battery comprising a positive electrode and a negative electrode,
The positive electrode comprises a positive electrode active material having a hollow structure having a shell portion and a hollow portion formed therein,
The positive electrode active material has the following conditions:
Particle porosity is 23% or more;
Includes a lithium transition metal oxide having a layered crystal structure, the lithium transition metal oxide contains at least one metallic element M _T of Ni, Co and Mn;
In the powder X-ray diffraction pattern using CuKα rays, the ratio (A / B) between the half width A of the diffraction peak obtained by the (003) plane and the half width B of the diffraction peak obtained by the (104) plane is 0. .7 or less; and
The content of the compound containing Li and CO ₃ is 0.2% by mass or less;
Satisfying the lithium secondary battery. The ratio (m _Li / m _MT ) of the molar number m _Li of lithium contained in the positive electrode active material to the total molar number m _MT of Ni, Co and Mn contained in the positive electrode active material is 1.10 or more. The lithium secondary battery according to claim 1. The positive electrode active material, W, further comprises at least one metal element M _A is selected from Cr and Mo,
The Ni contained in the positive electrode active material, is taken as 100 mol% of the total molar number _{m MT} of Co and Mn in a molar percentage content _{m MA} of the _{M A} is 0.05 mol% to 1 mol%, The lithium secondary battery according to claim 1 or 2. The lithium transition metal oxide, Ni as the M _T, containing Co and Mn, the lithium secondary battery according to any one of claims 1 to 3.   5. The lithium secondary battery according to claim 1, wherein the positive electrode active material has a through-hole penetrating the shell portion. 6.   The lithium secondary battery according to any one of claims 1 to 5, which is used as a power source for driving a vehicle.

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